Background: The rare CisAB blood genotype results in the inheritance of both the A and B blood types from a single parent. Several CisAB blood type phenotypes have been characterized that differ in their serological reactions and in the activities of the gene-encoded blood group A and B transferases. In this study, we conducted serological and genetic analyses of a Chinese family with four CisAB-carrying members. Study design and methods: A 10-year-old girl was suspected to have the CisAB blood type when forward blood typing indicated that she was blood type AB but reverse blood typing indicated that she was blood type A. An examination of this family identified that four of ten family members across three generations had a pattern of AB inheritance that could be explained only by the inheritance of the CisAB genotype. Blood samples from the family were tested serologically and genetically using PCR-SSP for the relevant CisAB alleles. The samples that were suitable for DNA analysis were sequenced for potential mutations. Results: Of the ten family members included in this study, four members showed mismatched forward and reverse blood grouping results, four members typed as blood group 0 and the remaining two members typed as blood group B. Among the four mismatched individuals, the propositus and her mother's mother were serologically typed as having the A(2)B(3) phenotype. The propositus' mother and her mother's brother were typed as having the A2B phenotype, and had the genotypes CisAB01/002 and CisAB01/B101, respectively. Further sequencing analysis of the four samples with the CisAB blood type revealed that a G(261) deletion occurred along with 297A > G, 467C > T, 6461 > A, 681G > A, 771C > T, 803G > C and 829G > C substitutions in exons 6 and 7 of CisAB01/002, while a 297A> G substitution occurred in exon 6 and 467C > T, 803G > C, 526C > G, 657C >T, 703G > A, 796C > A, 930G > A substitutions occurred in exon 7 of CisAB01/B101. Conclusion: A family was identified in which 4/10 family members across three generations had inherited the CisAB blood group. Two of the CisAB genotypes were further identified as CisAB01/002 and CisAB01/B101. The CisAB heterozygous alleles contributed to the different phenotypes that were observed. (C) 2015 Elsevier Ltd. All rights reserved.