Key Insights

• See whether your lung tumor carries a ROS1 gene fusion that drives growth and can change treatment options with your oncology team.
• Identify the exact fusion partner and related tumor markers that help explain symptoms, staging, and risk of spread.
• Learn how tumor genetics, smoking history, and sampling method can shape results and what they mean for your biology.
• Use findings to guide targeted therapy decisions and clinical trial eligibility in partnership with your clinician.
• Track changes over time to monitor response or emerging resistance using repeat tissue or plasma testing.
• Integrate results with other panels such as EGFR, ALK, BRAF, KRAS, MET, RET, NTRK, PD-L1, and tumor mutational burden for a complete view of your cancer.

What Is a ROS1 Fusion Test?

A ROS1 fusion test detects when the ROS1 gene has become abnormally joined to another gene in a lung tumor. This rearrangement creates a hybrid “on switch” that can fuel cancer cell growth. Testing is performed on tumor tissue from a biopsy or surgical sample, or on plasma using circulating tumor DNA (a liquid biopsy). Laboratories commonly use next-generation sequencing to read DNA or RNA and identify fusion breakpoints, fluorescence in situ hybridization to visualize the rearrangement in cells, and sometimes immunohistochemistry as a screening tool. Results are reported as fusion positive or negative, often with the name of the partner gene (for example, CD74‑ROS1) and technical details like assay sensitivity and, in some reports, variant allele fraction.

This test matters because ROS1 fusions are potent drivers in a small but important subset of non‑small cell lung cancers. Finding a fusion provides objective, actionable information about the tumor’s biology and signaling pathways. It helps explain how the cancer is behaving, supports precision treatment planning, and offers a measurable target to follow over time. In short, it connects molecular changes inside cells to real‑world outcomes like symptom control, response to therapy, and long‑term resilience.

Why Is It Important to Test Your ROS1?

ROS1 is a receptor tyrosine kinase. When it fuses with another gene, the kinase can become stuck in an active state that continuously sends growth signals. In lung adenocarcinoma, this single event can act like a master key for unchecked cell division, survival, and spread. A ros1 fusion test tells you if this is the circuit your tumor is using. The finding is most relevant at diagnosis of advanced non‑squamous non‑small cell lung cancer, at recurrence, or if the cancer stops responding after a period of control. Although ROS1 fusions are uncommon overall, they are enriched in adenocarcinomas and in people who have never smoked or smoked lightly, but they can occur in anyone. The result is not inherited; it arises within the tumor itself.

Zooming out, precision oncology is about matching the right strategy to the right biology. Regular molecular testing lets you measure rather than guess. It can detect early warning signs that a treatment is no longer working, reveal new resistance mechanisms, and show when a different approach may make more sense. The goal is not to “pass” or “fail.” It is to understand what is driving the cancer at this moment and adjust with your care team to improve control, reduce side effects, and support longevity, ideally using the least amount of intervention needed for the most benefit.

What Insights Will I Get From a ROS1 Fusion Test?

Reports typically state positive or negative for a ROS1 rearrangement. Some assays include the fusion partner, the exact breakpoints, and a quantitative estimate such as variant allele fraction in tissue or plasma. Laboratories set internal thresholds for calling a fusion, and results are interpreted against these cutoffs. In this context, “normal” means no fusion detected on that sample, while “positive” indicates a ROS1‑driven signaling pathway that may be targetable. Context matters: sample quality, tumor content, and the technology used can influence detection.

If no fusion is found, it suggests your tumor is powered by different pathways, and other drivers should be evaluated. If a fusion is present, it indicates oncogenic ROS1 activity that can shape treatment decisions with your oncologist. Variation in quantitative metrics can reflect tumor heterogeneity, prior therapy, or the proportion of tumor DNA in the sample.

Higher or lower allele fractions do not map cleanly to “better” or “worse.” A low fraction in plasma may simply reflect limited DNA shedding into the bloodstream. Likewise, immunohistochemistry signal intensity does not replace molecular confirmation. A positive result does not diagnose cancer on its own; it adds a precise layer of biology to a diagnosis already established by imaging and pathology.

The real power is in pattern recognition over time. In tissue, fusion status anchors first‑line planning. In plasma, repeat testing can help track response or reveal resistance alterations that emerge under treatment. When integrated with other biomarkers and your clinical picture, these patterns support earlier detection of change and more personalized planning.