GAPDH and rectal neoplasm: Five promising reference genes GAPDH, Pumilio RNA binding family member 1 (PUM1), beta-2-microglobulin (B2M), ribophorin I (RPN1), and phosphomannomutase 1 (PMM1) were selected from different publications [7,9-10] to assess their expression stability in rectal tumors as part of the screening phase.