The cis-AB inheritance of the ABO blood group is shown with two family cases. In Family 1, the father was phenotypically A2B3. The mother was O (OO) and the daughter, one of the children, was phenotypically A2B3. This can be explained by assuming that the father’s ABO genotype was cis-AB/O, and that the daughter inherited the cis-AB allele from her father and the O allele from her mother, resulting in the genotype of cis-AB/O, the same genotype as her father. In Family 2, the ABO phenotypes of the father, mother, and son were A1, A1B3, and A2B3, respectively. This inheritance can be explained by assuming that the mother’s genotype was cis-AB/A1, and the son inherited the O allele from his father and the cis-AB allele from his mother to be cis-AB/O.