MAF1 and glioblastoma: The reduction in Maf1 is observed before the onset of tumors in these tissues [14], [15] indicating that the changes in Maf1 are concurrent with the loss of Pten. Consistent with a reduction in Maf1 upon loss of PTEN, induction of PTEN expression in PTEN-deficient human glioblastoma U87 cells resulted in an increase in Maf1 expression and required the phosphatase activity of PTEN (Fig. 1B).