While at Cambridge University, Grainger founded life sciences companies including FingerPrint Diagnostics (2001), and Funxional Therapeutics (2005). FingerPrint Diagnostics merged with SmartBead Technologies to form Pronostics, a molecular diagnostics company, in 2006. Funxional Therapeutics, based on an anti-inflammatory drug candidate spun-out from Grainger's Cambridge University lab, became an Index Ventures portfolio company where Grainger also served as chief scientific officer until it was sold to Boehringer Ingelheim in 2012.