PPIA and cancer: First, we assessed the expression stability of a set of 32 frequently used RGs in 12 selected cancer types: ACTB, ALAS1, B2M, CDKN1A, G6PD, GAPDH, GUSB, HBB, HMBS, HPRT1, HSP90AB1, IPO8, LDHA, NONO, PGK1, POP4, PPIA, PPIH, PSMC4, PUM1, RPL13A, RPL30, RPLP0, RPS17, RPS18, SDHA, TBP, TFRC, UBC, YWHAZ, TUBB, RPN1.