Diagnostic testing company Myriad Genetics Inc. said Thursday that it loaned $25 million to Crescendo Bioscience Inc. as part of a deal that includes an exclusive three-year option for Myriad to buy Crescendo.

Myriad said that acquisition could expand its portfolio of molecular diagnostic products to cover autoimmune disorders such as rheumatoid arthritis. Myriad currently specializes in oncology, preventative care, urology, dermatology and neuroscience products.

It said Crescendo has developed a diagnostic test for rheumatoid arthritis that satisfies and unmet need.

The exact terms of the deal weren't disclosed, but Myriad said it can exercise its option if Crescendo Bioscience, based in South San Francisco, attains a minimum revenue level during the three-year term. The purchase price in that scenario would be determined by a formula based on Crescendo's revenue growth.

If Crescendo Bioscience doesn't attain the minimum revenue, Myriad would have a one-time right to exercise the option for a fixed price. In either case, the purchase would be all in cash.

Myriad said its $25 million loan to Crescendo Bioscience is for six years at an interest rate of 6 percent.

Shares Myriad closed down 23 cents at $18.96.