Silicon Valley start-up Counsyl is selling a test that it says can tell couples whether they are at risk of having children for 100 inherited diseases, including rare inherited diseases.

Some genetic testing of prospective parents is done now, but only for a few diseases like cystic fibrosis and Tay-Sachs, and only for certain ethnic groups. Each test can cost hundreds or even thousands of dollars. Counsyl’s test costs $349 for an individual or $698 for a couple.