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). This evidence concerns the gene PIK3R1 and glioblastoma.