Many genes act as both tumor suppressor gene (TSG) and proto-oncogene depending on cellular context and cancer type. Nemo-like kinase (NLK) encoding a serine/threonine kinase, Yin Yang 1 (YY1) encoding a zinc-finger transcription factor and PA2G4 encoding an ErbB3 binding protein have both of these two opposing functions. In the present study, we analyzed NLK, YY1 and PA2G4 frameshift mutations in sporadic GC and CRC with high microsatellite instability (MSI-H). Also, regional intratumoral heterogeneity (ITH) of frameshift mutations of these genes was analyzed in CRCs. We found frameshift mutations of NLK, YY1 and PA2G4 in CRC and GC with MSI-H (17/132: 12.9%), but not in those with MSS (0/90). Two (12.5%), one (6.3%) and one (6.3%) CRC (s) of the 16 CRCs exhibited ITH of NLK, YY1 and PA2G4 mutations among the 4–7 regions, suggesting that ITH of the frameshift mutations might be frequent in the CRCs. These results suggest that frameshift mutations of NLK, YY1 and PA2G4 along with the ITH might contribute to MSI-H cancer pathogenesis.