ROS1 rearrangement

ROS1 rearrangement is a gene fusion biomarker found in about 1% to 2% of non-small cell lung cancers (NSCLC). It means the ROS1 gene has become abnormally joined to another gene, creating a fusion protein that can drive cancer growth. The most important practical point is that a ROS1-positive tumour may respond to targeted ROS1 inhibitor treatment, so testing can directly affect treatment choices in advanced lung cancer.

What it is

ROS1 is a gene that encodes a receptor tyrosine kinase. In some cancers, especially a small subset of NSCLC, part of the ROS1 gene breaks and fuses with part of another gene. This is called a ROS1 rearrangement or ROS1 fusion. The result is an abnormal signaling protein that stays switched on and promotes tumour cell survival and proliferation.

In lung cancer, ROS1 rearrangements are most often discussed in adenocarcinoma, and they are reported more often in people who are younger and have little or no smoking history, although they can occur in other patients too. ROS1 fusions are considered an actionable biomarker, meaning the result can guide therapy.

A few common ROS1 fusion partners have been described, including CD74, SLC34A2, EZR, and SDC4, but many partners exist. For most patients, the key issue is not the exact partner name but whether a validated test confirms that the tumour is ROS1-positive.

Term	What it means
ROS1 rearrangement	A structural change involving the ROS1 gene
ROS1 fusion	ROS1 joined to another gene, creating an abnormal driver protein
ROS1-positive	The tumour tested positive for a ROS1 fusion/rearrangement
ROS1 fusion testing	Lab testing done on tumour tissue or blood to look for this biomarker

How it works

ROS1 belongs to the receptor tyrosine kinase family. When ROS1 is fused to another gene, the fusion can keep the kinase domain active without normal control. This abnormal activation triggers downstream pathways involved in cell growth, survival, and spread.

This matters because targeted drugs can block ROS1 signaling. In ROS1-positive NSCLC, these medicines may shrink tumours or slow progression more effectively than non-targeted treatment in the right setting. That is why ROS1 testing is part of biomarker work-up for many patients with advanced non-squamous NSCLC.

Different fusion partners may affect biology in subtle ways, and research has explored whether some partners respond differently to treatment. So far, evidence suggests many ROS1 fusion partners are clinically targetable, but the exact impact of partner type is still being studied.

Evidence and uses

The main clinical use of ROS1 rearrangement testing is in non-small cell lung cancer, especially advanced lung adenocarcinoma. Reviews in the medical literature place ROS1 rearrangements in roughly 0.9% to 2.6% of NSCLC cases, with many sources summarising the frequency as about 1% to 2%.

Why test for it? Because a positive result can open the door to ROS1-targeted therapy. ROS1 inhibitors such as crizotinib and newer agents have shown meaningful activity in ROS1-positive NSCLC. In practice, this makes ROS1 one of the established molecular targets in lung cancer care.

Testing is usually considered alongside other driver biomarkers such as EGFR, ALK, BRAF, MET exon 14 skipping, RET, NTRK, and KRAS, depending on the clinical setting and local protocols. In India, many cancer centres now use multigene molecular panels for advanced NSCLC when feasible, because a single biopsy sample may need to answer several treatment-defining questions.

ROS1 testing may be done by one or more of these methods:

1. Immunohistochemistry (IHC): a screening test that looks for ROS1 protein expression in tumour tissue.
2. Fluorescence in situ hybridisation (FISH): historically an important confirmatory method for detecting rearrangements.
3. Next-generation sequencing (NGS): can identify ROS1 fusions and often the fusion partner; increasingly useful when tissue is limited.
4. Reverse-transcriptase PCR: can detect known fusion transcripts but may miss uncommon partners.
5. Liquid biopsy: blood-based testing may help in some cases, especially when tissue is hard to obtain, but sensitivity can be lower than tissue testing.

Test method	Main strength	Main limitation
IHC	Fast, relatively accessible screening	Positive result may need confirmation
FISH	Established method for rearrangements	Requires expertise; may miss some complexity
NGS	Broad profiling, detects multiple biomarkers at once	Cost, turnaround time, and tissue quality matter
PCR	Sensitive for known fusions	Can miss rare or novel partners
Liquid biopsy	Less invasive	A negative result may not rule out ROS1 fusion

Diagnosis / how it's measured

ROS1 rearrangement is not diagnosed from symptoms or scans. It is measured in a pathology or molecular laboratory using a tumour sample from biopsy or surgery, and sometimes with circulating tumour DNA from blood.

The pathology report may say ROS1 rearrangement detected, ROS1 fusion positive, or may name the exact fusion, such as CD74::ROS1. A negative result should be interpreted in context. If tissue is very small, degraded, or contains few tumour cells, false-negative results are possible. That is one reason oncologists may request repeat testing, reflex testing by another method, or broader NGS.

For patients and families, it is reasonable to ask four practical questions:

1. Was ROS1 tested at all?
2. Which method was used?
3. Was the sample tissue or blood?
4. If negative, was the sample adequate and were other driver mutations also checked?

When to see a clinician

Discuss ROS1 testing with an oncologist if you or a family member has newly diagnosed advanced NSCLC, especially adenocarcinoma, or if treatment decisions depend on molecular profiling. If a pathology report mentions ROS1, fusion, rearrangement, or driver mutation, ask the treating team to explain what it means for treatment.

You should also ask for review if results are unclear, if only limited biomarker testing was done, or if a liquid biopsy was negative but suspicion remains high. In many centres, a medical oncologist, molecular pathologist, and thoracic tumour board help interpret complex results.

Limitations and open questions

ROS1 rearrangement is an important biomarker, but testing has limits. Not every assay performs the same way, and no single method is perfect in all situations. Small biopsies, low tumour content, RNA degradation, and technical differences between laboratories can affect results.

Another open question is how much the fusion partner changes prognosis or response to specific ROS1 inhibitors. Some studies suggest differences, while others show broadly comparable benefit across common partners. Evidence is still evolving.

Resistance is another major issue. Even when a ROS1-targeted drug works well at first, cancers can later develop secondary resistance mutations or progress in the brain or elsewhere. This is why repeat molecular testing at progression may be useful.

Finally, access is uneven. In India and elsewhere, availability of NGS, turnaround time, cost, and tissue handling can affect whether ROS1 testing is done early and accurately. A negative or incomplete report does not always mean the biomarker is truly absent. When treatment decisions are major, it is reasonable to ask whether additional molecular testing is needed.