Inherited mosaicism affecting the ABO blood groups.

Blood samples from two families and three other unrelated people contain a mixture of red blood cells of two different ABO types. Previously described causes of red blood cell mosaicism have been excluded from responsibility. Other blood group systems are not involved. The phenotypes, which have been designated ABO mos, are inherited through a variant allele at the ABO locus and appear to arise from a change in regulation of an ABO gene of some somatic cells.