PIK3R1 and neoplasm: Mutations in the three major effectors of the PI3K pathway: PTEN, the main tumor suppressor of the pathway; PIK3CA, encoding the PI3K catalytic subunit p110α; and PIK3R1, encoding the PI3K regulatory subunit p85, and only rare mutations of MTOR, were present in three quarters of the glioblastoma cases (Figure 4a and Table 3).