Cancer cells mutate as they spread, increasing the chance that some may become resistant to treatment. A diagram showing how the genetic mutation of the primary tumor affects metastic sites.

Results from genomic testing of a cancer patient’s biopsy tissue can help guide treatment decisions, depending on the types of gene mutations found. But as it spreads or stops responding to treatment, the cancer changes and develops new mutations. Insights gained from the biopsy of the primary tumor may no longer be relevant in determining next steps.