PIK3CB and prostate cancer: However, early signals of antitumour activity with RECIST responses were observed, suggesting that patients with tumours harbouring concomitant genetic alterations in PIK3CB may benefit from PI3Kβ blockade; exploratory genomic analyses of tumour samples from 10 prostate cancer patients revealed a PIK3CB L1049R mutation in a tumour specimen from a patient who remained on study for 33 weeks, and an increased tumoural PIK3CB gene copy number in a patient who achieved a RECIST partial response.