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What happened

Myriad Genetics (NASDAQ:MYGN) fell 11.6% today, after Tesaro (NASDAQ: TSRO) presented data at the European Society for Medical Oncology that suggest Myriad Genetics' companion diagnostic test might not be necessary before treating ovarian-cancer patients with Tesaro's drug candidate niraparib.

So what

Niraparib works by damaging tumor cells' DNA, so Tesaro used Myriad Genetics' test to determine if patients had mutations in the BRCA genes, which causes the cells to be unable to repair damaged DNA. Myriad also has a test called myChoice HRD, which identifies tumors that have lost their ability to repair DNA damage even if they don't have BRCA mutations.

The problem for Myriad Genetics is that niraparib seems to help all patients, even ones that were HRD negative, arguing that all patients should get niraparib. That's good news for Tesaro, and the reason it ended up 18.8% today, but it's bad news for Myriad Genetics, since it argues the test is unnecessary.

Now what

Myriad's saving grace may come from the fact that while the drug appears to be helping patients regardless of what the myChoice HRD test read out, patients with HRD-positive tumors benefited substantially more with their tumors kept at bay for 12.9 months, compared with 6.9 months for HRD-negative tumors and 3.8 months for the control group.

While doctors may not need Myriad Genetics test to tell whether patients will benefit from niraparib, patients may want the test to know how much the drug is likely to help them. Six months is a big difference for a cancer patient.

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