An overview of the genes and their protein products for ABO-Glycosyltransferase, Rhesus (RHD and RHCE), Kell, Kidd and Duffy will be given. The various alleles of these loci are due to single point mutations in the most cases, with exemption of Rhesus, where larger gene rearrangements ("gene conversions") also have taken place. A new ABO "O allele", caused by a single point mutation near the active center of the enzyme, will be presented. Polymerase Chain Reaction using sequence specific priming (PCR-SSP) was used to type for all of the presented alleles. The principle of this method relies on the relative inability of Taq-Polymerase to start from 3′-mismatched primers. Hence, it offers a powerful tool to detect single point mutations. To use the same principle for large gene rearrangements is even more facile. The presented DNA typing method (PCR-SSP) were employed for various objectives: Prenatal typing for RHD, or Kell are the most commonly demanded. But molecular typing may also be employed, to confirm the presence of a weak Fyb variant (FY*X), to define weak RHD antigens (Du), or to tell the original blood groups from a polytransfused patient. This shows, that molecular typing techniques offer an interesting addition to well established standard serology. Presently, with respect to blood groups, molecular biologists and medical doctors seem to be challenged by the identification of un- or weakly expressed alleles. Also, the "non-presence" of RHD - a hypothetical RHd allele - and its molecular background is of big interest with clinical implication.