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DNA from the double short tandem repeat (STR) system MBP (locus 18q23-pter) was amplified by the polymerase chain reaction (PCR) and the two polymorphic repeat systems were separated by cutting with the restriction enzyme NlaIII. The lengths of the DNA fragments of the two MBP STR systems MBP-A and MBP-B were analyzed by vertical electrophoresis in polyacrylamide gels followed by silver staining. DNA samples from 112 unrelated Danes, 140 unrelated Greenland Eskimos, and 88 Danish mother/child pairs were analyzed. The distributions of MBP phenotypes were in Hardy-Weinberg equilibrium in both the Eskimo and Danish populations. Significant differences were observed between the distribution of fragments ('alleles') in Greenland Eskimos and in Danes. The allele MBP-A7 was considerably more frequent in Eskimos (0.2214) than in Danes (0.0775) and also the allele MBP-B9 was considerably more frequent in Eskimos (0.225) than in Danes (0.06). Strong gametic associations were found between fragments from MBP-A and MBP-B series in both Danes and Eskimos. Some of the associations were different in Danes and Eskimos. In the 88 Danish mother/child pairs, the segregation of the MBP genotypes were in accordance with a genetic model of co-dominant inheritance and no mutation was found. Two MBP STR regions with irregular structures were sequenced. One fragment had a single base G to A transition at position 124 in the primer binding region between the MBP-A and MBP-B regions. In the other fragment, a deletion starting at position 117 and including the primer binding region between MBP-A and MBP-B regions was found.