biomarker

Loss of one copy each of chromosome arms 1p and 19q, together with an IDH mutation, defines oligodendroglioma in the WHO 2021 classification. It predicts a slower course and a good response to chemotherapy, and separates oligodendroglioma from astrocytoma.

AKT1 E17K is a single hotspot mutation, in about 3 to 5 percent of hormone-receptor-positive breast cancers, that switches on the AKT kinase directly. It is one of the three alterations that qualify a patient for capivasertib.

An ALK fusion, usually EML4::ALK, is a swapped piece of chromosome 2 that turns the ALK kinase on permanently in about 4 percent of lung adenocarcinomas. Seven ALK inhibitors are approved for it, including alectinib after surgery.

AR-V7 is a shortened androgen receptor that lacks the part hormone drugs bind, so it stays active without testosterone. Found in circulating tumour cells, it predicts poor response to abiraterone and enzalutamide, but no label uses it yet.

B7-H3 is an immune checkpoint protein overexpressed on small-cell lung, prostate and many solid tumours. Ifinatamab deruxtecan is in phase 3 for small-cell lung cancer without an expression cut-off; nothing is approved.

BCMA is the plasma-cell antigen behind the myeloma bispecifics and CAR-T cells. None of their labels requires a BCMA test; expression is near-universal, and loss through BCMA gene deletion is a documented escape route.

T315I is the gatekeeper mutation in the ABL1 kinase that defeats imatinib, dasatinib, nilotinib and bosutinib. Ponatinib and asciminib are the two drugs approved for it.

The BCR::ABL1 fusion is the Philadelphia chromosome that defines chronic myeloid leukaemia and some acute lymphoblastic leukaemia. Its transcript level in blood, on the international scale, is how response to tyrosine kinase inhibitors is measured and when treatment can be stopped.

Class II and III BRAF mutations sit outside codon 600 and signal as pairs (class II) or by leaning on RAS (class III). Approved BRAF inhibitors do not work on them, and no drug is yet approved for them; pan-RAF inhibitors are in trials.

A BRAF fusion joins another gene to the BRAF kinase, most often KIAA1549 in paediatric low-grade glioma. Tovorafenib is approved for children whose relapsed low-grade glioma carries a BRAF fusion or V600 mutation.

V600E is the BRAF change that drives half of melanomas and many thyroid, bowel, lung and brain tumours. BRAF plus MEK inhibitors are approved for it in melanoma, lung, thyroid and, tumour-agnostically, any solid tumour; in bowel cancer it is treated with encorafenib and cetuximab.

DNA repair genes. Inheriting a broken copy raises breast and ovarian cancer risk, but tumours that lose them become uniquely vulnerable to PARP inhibitors and platinum.

c-Met overexpression is a stain, not a gene change: strong (3+) membrane staining in at least half of tumour cells. It selects telisotuzumab vedotin, an antibody-drug conjugate, in previously treated non-squamous lung cancer.

CD19 is the B-cell surface protein that blinatumomab, CAR-T cells and several antibodies aim at. Blinatumomab's label requires CD19-positive leukaemia; the CAR-T labels assume it, and losing CD19 is the commonest way the disease escapes.

CD20 is the B-cell antigen rituximab made famous. Rituximab and obinutuzumab labels are written for CD20-positive lymphoma and leukaemia; the newer CD20 x CD3 bispecifics assume it.

CD22 is a second B-cell antigen, kept even when CD19 is lost. Inotuzumab ozogamicin is labelled for CD22-positive acute lymphoblastic leukaemia.

CD30 is the antigen of Hodgkin Reed-Sternberg cells and anaplastic large cell lymphoma. Brentuximab vedotin is labelled for Hodgkin lymphoma without a CD30 test and for peripheral T-cell lymphomas that express it.

CD33 is a myeloid antigen on the blasts of about 90 percent of acute myeloid leukaemias. Gemtuzumab ozogamicin's label requires CD33-positive disease.

CD38 is on almost every myeloma cell, so daratumumab and isatuximab are given without testing for it. The stain matters afterwards: CD38 falls after treatment and can confuse flow cytometry and blood typing.

The classic 'CEA' tumour marker measured in blood, also present on the cell surface where ADCs can reach it.

CEACAM5 is the cell-surface form of the tumour marker CEA, high on many lung and bowel cancers. Tusamitamab ravtansine was tested in CEACAM5-high lung cancer but failed its phase 3 and no approval exists.

Claudin 18.2 is a tight-junction protein exposed on stomach cancer cells. Zolbetuximab requires at least 75 percent of tumour cells to stain moderately or strongly, the strictest expression threshold in any current label.

A ctDNA MRD test looks for the tumour's own mutations in blood after surgery. Detection predicts relapse months before scans, and in 2026 the FDA approved Signatera as the companion test selecting bladder cancer patients for adjuvant atezolizumab.

DLL3 sits on the surface of about 85 percent of small-cell lung cancers and almost no normal adult tissue. Tarlatamab is given without a DLL3 test.

dMMR means one of the four mismatch repair proteins is missing from the tumour cell nuclei on a stain. It is the tissue-level twin of MSI-high and opens checkpoint immunotherapy in endometrial, bowel and many other cancers.

An exon 19 deletion removes a few amino acids from the EGFR kinase and leaves it switched on. With L858R it makes up about 85 percent of EGFR-mutant lung cancer and is the classic gate for osimertinib and the other EGFR inhibitors.

Exon 20 insertions add amino acids after the C-helix of EGFR and make the kinase resistant to the usual EGFR tablets. They account for about a tenth of EGFR mutations and have their own drugs: amivantamab with chemotherapy and sunvozertinib.

L858R is a single letter change in exon 21 of EGFR that keeps the kinase active. It is the second commonest sensitising mutation and shares every EGFR inhibitor label with the exon 19 deletion.

T790M is the gatekeeper mutation that lung cancers acquire to escape first- and second-generation EGFR inhibitors. Finding it, in tissue or blood, is the historic gate for osimertinib after an earlier EGFR drug.

ER status is whether the tumour's cells carry the oestrogen receptor, read by staining nuclei. One percent or more of stained nuclei is positive and means hormone-blocking treatment can work; 1 to 10 percent is 'low positive' and behaves more like negative.

ESR1 mutations arise in the oestrogen receptor's ligand-binding domain after aromatase inhibitor treatment, letting the receptor work without oestrogen. They are usually found in a blood test and select the oral SERDs elacestrant and imlunestrant.

FcRH5 is a third myeloma surface target after BCMA and GPRC5D. Cevostamab, the FcRH5 x CD3 bispecific, is in phase 3; no approval or threshold exists yet.

FGFR2 fusions occur in 10 to 15 percent of intrahepatic bile duct cancers and almost nowhere else. Pemigatinib and futibatinib are approved for them after a first chemotherapy.

FGFR3 point mutations and TACC3 fusions drive about 15 to 20 percent of advanced bladder cancers. Erdafitinib is approved for tumours with these 'susceptible' alterations after one prior treatment.

FLT3-ITD is a duplicated stretch of the FLT3 receptor gene found in about a quarter of acute myeloid leukaemias; it makes relapse more likely and is treated with midostaurin, quizartinib or gilteritinib.

FLT3-TKD mutations are point changes in the kinase's activation loop, found in about 7 percent of acute myeloid leukaemias. Midostaurin and gilteritinib labels cover them; quizartinib's does not.

Folate receptor alpha is a surface protein on most high-grade serous ovarian cancers. Mirvetuximab soravtansine requires FRα-positive disease, scored as at least 75 percent of cells with moderate or strong staining on the Ventana FOLR1 assay.

A germline BRCA1 or BRCA2 variant is inherited and present in every cell, found by a blood test. It selects PARP inhibitors in breast, ovarian, pancreatic and prostate cancer and tells relatives they may carry it too.

GPRC5D is a receptor on myeloma cells and in hair follicles, nails and taste buds. Talquetamab targets it without any test, and the side effects on skin, nails and taste follow from where it is expressed.

H3 K27M is a single change in a histone that defines diffuse midline glioma, including the brain-stem tumour DIPG. In 2025 dordaviprone became the first drug approved for tumours carrying it.

A HER2 mutation is a change in the gene's kinase domain, most often an exon 20 insertion, found in about 2 to 3 percent of lung adenocarcinomas. It is a different thing from HER2 amplification or overexpression, and it selects trastuzumab deruxtecan and, since 2025, zongertinib in lung cancer.

IHC 0 is no HER2 staining, or faint staining in 10 percent or fewer cells. It is HER2-negative, but the label now separates true zero from 'IHC 0 with membrane staining', the ultralow group that trastuzumab deruxtecan can treat in hormone-receptor-positive breast cancer.

IHC 1+ is faint, incomplete HER2 staining. It was called HER2-negative for twenty years; since 2022 it is the larger half of HER2-low, which trastuzumab deruxtecan treats in breast cancer.

IHC 2+ is the in-between HER2 result: moderate staining that cannot be called positive or negative by eye, so the laboratory runs an ISH gene test. 2+ with amplification is HER2-positive; 2+ without it is HER2-low.

IHC 3+ means strong, complete membrane staining for HER2 in more than 10 percent of tumour cells. It is HER2-positive without needing a gene test and is the gate for trastuzumab, its combinations and antibody-drug conjugates in breast, stomach, biliary and, since 2024, any solid tumour.

ISH counts copies of the HER2 gene in each tumour cell. A ratio of 2 or more against the chromosome 17 control, or 6 or more copies per cell, is amplified and HER2-positive whatever the protein stain showed.

HER2-low is not a new stain but a new reading of the old one: 1+ or 2+ without gene amplification. It covers about half of breast cancers and makes them eligible for trastuzumab deruxtecan.

HER2-ultralow is an IHC 0 result with a trace of membrane staining in a few cells. Since 2025, in hormone-receptor-positive breast cancer that has stopped responding to hormone therapy, it is enough for trastuzumab deruxtecan.

HER3 is a signalling partner of EGFR and HER2 present on most lung and breast cancers. Patritumab deruxtecan was studied in EGFR-mutant lung cancer without a HER3 threshold, and no approval exists.

HLA-A*02:01 is the commonest tissue-type molecule in people of European descent and the one the first T-cell receptor therapies were built for. Tebentafusp and afamitresgene autoleucel work only in patients who carry it, so a blood HLA test comes before the tumour test.

HRD-positive means the tumour's genome carries the scars of failed double-strand break repair (or a BRCA mutation), measured as a genomic instability score. In ovarian cancer it selects niraparib, and olaparib with bevacizumab, as first-line maintenance.

IDH1 R132 mutations turn a metabolic enzyme into a producer of the oncometabolite 2-HG. They define lower-grade gliomas and occur in acute myeloid leukaemia and bile duct cancer, each with an approved IDH1 inhibitor.

IDH2 mutations at codons 140 and 172 do the same job as IDH1 R132, producing 2-HG. Enasidenib is approved for relapsed AML with them, and vorasidenib for grade 2 gliomas with either IDH gene mutated.

Ki-67 is the percentage of tumour cells that are dividing. In neuroendocrine tumours it sets the grade; in breast cancer a 20 percent cut-off was briefly a condition of adjuvant abemaciclib, then dropped from the label in 2023.

KIT D816V is the mutation behind almost every case of systemic mastocytosis. It makes the disease resistant to imatinib, and it is detected by a highly sensitive blood PCR; avapritinib treats the disease whether or not the mutation is confirmed.

KRAS G12C swaps glycine 12 for cysteine and was the first KRAS mutation a drug could grip. Sotorasib and adagrasib are approved for it in lung cancer, and with an EGFR antibody in bowel cancer.

G12D is the commonest KRAS mutation, especially in pancreatic cancer, and has no approved drug yet. In bowel cancer any KRAS or NRAS mutation is a reason not to give EGFR antibodies, which is where the approvals sit.

MET amplification means extra copies of the MET gene, either as a primary driver in a few lung cancers or as the escape route after EGFR inhibitors. No label yet selects on it; trials define it by FISH ratio or copy number.

MET exon 14 skipping is a splice-site change that lets the MET receptor escape degradation and keep signalling. It is found in 3 to 4 percent of lung cancers and is treated with capmatinib or tepotinib.

Methylation of the MGMT promoter switches off the repair enzyme that undoes temozolomide's damage. Methylated glioblastomas live longer on temozolomide; unmethylated ones gain little, and trials now use the result to spare or intensify chemotherapy.