GAPDH and glioblastoma: However, if we compared raw CT values of tumor versus non-neoplastic samples, as suggested by Ohl F. and collaborators (2005), significant differences in gene expression between GBM and normal white matter were found for ACTB (Student's t test, P = 0,016), GAPDH (P = 0,006), GUSB (P = 0,005) and 18S rRNA (P = 0.012).