Cis AB
Cis AB is a type of rare mutation in the ABO gene. It happens when the transferase allele contains a mix of amino acids from either A or B alleles, producing a bifunctional enzyme that can produce both types of antigens, usually with one weaker than the other. This results in a serum test result much like the standard, separate (trans) AB phenotype, although the weaker antigen can occasionally fail to be detected. It complicates the basic inheritance pattern (as the allele comes from one parent only) and blood-transfusion compatibility matching for ABO blood typing. Different DNA  mutations of either type A or Type B alleles change amino acids in enzyme transferase A or B, homologous enzymes differing in only four of 354 amino acids (R176G, G235S, L266M, and G268A). A single change in ABO gene DNA could switch type B to type A and then, a new hybrid enzyme can produce both weak B and A2 (in serum test, A2B, and A2B3). The most common mutation is an A105 allele variation in exon 7 nucleotide position G803C changing glycine (type A) to alanine (type B). Another 8 alleles are reported in BGMUT, most recently  in China and Taiwan. Some Cis-AB carriers need components like washed red blood cells or autotransfusion of serum and blood. Cis-AB type was studied first in humans in Japan (Shikoku Island) and South Korea (Gwangju area) where this rare type is more common (Chinese coast provinces and Taiwan also), although it is seen in a few European families (from France, Spain, Belgium, Germany, and Poland). In the year 2004, the American Red Cross described a family in Nebraska with a father of type cis-AB negative, a mother type O and their baby of type cis-AB. Since  then, several cases in the United States have been reported. Antigen expression is weaker than A1 or B. Scenarios. When one parent carries a Cis AB allele, the other allele can be any of O, A or B. The phenotype of this parent is always AB, but the children will inherit either the AB or the other allele from this parent. ABO inheritance is generally derived assuming the children are not the rare Bombay phenotype, which would require both parents to be carriers of it. Real life implications. Maternity and paternity disputes. When testing paternity or maternity by ABO blood  group alone, it is possible to have a paradoxical result in the rare instance that a cis-AB genotype is involved. For example, (scenario 1.1 above) a child of a cis AB individual (who will test as a regular AB phenotype) and an O individual will be either AB or O instead of the usual A or B (see diagram above). Differential diagnosis. If the child of an AB and an O individual is O (the green-colored offspring in the scenario 1 image above), then a rare alternative possibility is that the parents were carriers (heterozygous) for the Bombay  phenotype (Hh) and the child is a Bombay (hh) homozygous by genotype thus expressing Bombay phenotype also called O where, despite the presence of the ABO alleles, the substrate from which those antigens are made is not made and thus A, B and even O antigen are not expressed at all and are completely absent from the red cells.