Recombination within the class III region by a double cross over event.
Analysis of class I, class II, and class III gene products of the human MHC in a Caucasoid family with four children gave evidence for a double crossing over event in monozygotic twins between the C2, Bf/C4A, C4B gene loci. In view of the small genetic distance between C2 and C4, a point mutation within the Bf locus must be considered alternatively.