Generally, a group AB person cannot be the parent of a group O child, nor can a oup O person be the parent of a group AB child. But, rarely cases of an A2B child born to parents of group A2B and O or of group A2B and A1 were observed, and the most entertaining explanation for those cases were inheritance of A and B on one chromosome. For this unusal inheritance, the notation "cis-AB" was proposed. Recently, thors experienced two family cases of cis-A2B and were reported with a brief review of literatures.