The latest valuation is $5B, according to the WSJ[0]. The latest (series G) round was led by Fidelity, but Founders Fund, ARTIS Ventures, and SV Bank also participated in it[1]. Also, there's actually talk of Fidelity bringing sanity to startup valuations[2,3], rather than pumping them up even higher.

You're right that Stemcentrx, unlike Theranos or Nootrobox, doesn't smell of SV hucksterism. However, rather than comparing Stemcentrx to Theranos or Nootrobox, compare them to OncoMed, Verastem, and Stemline, which have current public valuations of $700M, $65M, and $116M, respectively. And none of them ever exceeded $1.2B in their public valuations.

0: http://blogs.wsj.com/venturecapital/2015/09/16/peter-thiel-b...

1: https://www.crunchbase.com/funding-round/7db717567346f07e604...

2: http://fortune.com/2015/11/11/snapchat-isnt-the-only-startup...

3: http://fortune.com/2015/11/12/fidelity-marks-down-tech-unico...