HPRT1 and brain neoplasm: In each individual experiment using individual housekeeping genes, CNS-NB samples showed significantly elevated PLK4 expression levels when compared to non-embryonal brain tumors (LGG) (GAPDH p = 0.0016; HPRT1 p < 0.0001; HMBS p = 0.0116) (Figure 3A–C).