Three years after it launched a bio fund, venture firm Andreessen Horowitz is finding opportunities in what general partner Vijay Pande calls "dark data."

The siloed nature of medical records means that valuable data often remain hidden from doctors, researchers and patients, making it difficult for people to get the targeted care they need at the most critical times.

For his latest bet, Pande is backing former Apple executive Anil Sethi, who has spent his career trying to unlock patients' health data from clunky electronic medical record systems and get it easily into the hands of those who most need it.

Sethi is the founder of Ciitizen, an early-stage start-up that's focused on helping people with cancer access their data. It's a deeply personal effort for Sethi, who left Apple late last year to care for his sister, Tania, as she was dying from breast cancer.

"When Tania was sick, her doctors wanted to know a few things, like her labs," Sethi said in an interview. "These things were super important but not always easy to access."

Sethi arrived at Apple in 2016, after his previous personal health start-up, Gliimpse, was acquired to help the iPhone maker bring medical records to the masses. Ciitizen has just raised $3 million in a funding round led by Andreessen Horowitz, which has raised two bio funds, including a $450 million pool in December.