Autonomous vehicle startup Zoox is in talks with Japanese investment firm SoftBank to raise more funding. We’ve confirmed the news, which was first reported by Axios.

We’re hearing the discussions are in the early stages and nothing has been finalized, but that SoftBank has been interested in Zoox for a while. The startup is also talking to other unnamed potential investors.

Zoox presently has a valuation above $1.5 billion and previously raised capital from Draper Fisher Jurvetson, Lux Capital and others. Axios is reporting that the SoftBank conversations have been in the $3 billion to $4 billion valuation range.

Building vehicles is a capital-intensive business, so it’s no surprise that Zoox would seek more funding. And SoftBank’s Vision Fund has about $100 billion to invest, so the team is looking for opportunities to write big checks.

Zoox has been quietly building self-driving cars, sharing little detail about its plans. We’re hearing that the group has explored creating an Uber-like on-demand service.

The race to build self-driving cars has been heating up. Uber and Alphabet’s Waymo are amongst the many companies that have been building prototypes.

Zoox was founded in 2014 by Jesse Levinson, who came from Stanford’s self-driving car group, and Tim Kentley-Klay, an entrepreneur from Australia. The Menlo Park-based startup has been hiring employees from companies like Tesla, Alphabet and Apple.

We’ve reached out to Zoox for comment.