PHGDH and glioblastoma: U251 human glioblastoma cells were selected as a model cell system to mimic a heterozygous condition because of the documented endogenous expressions of the PP enzymes (15.28 ± 1.91, 3.06 ± 0.38 and 3.63 ± 1.34 ng per 105 cells for PHGDH, PSAT and PSP, respectively) [22].