[MNSs system, erythrocyte membrane and false paternity exclusion]

The modification of the red cell membrane first observed by Darnborough and co-workers in En (a-) and EnaEn heterozygous persons, and characterized by a) exaggerated agglutinability of the cells by various serological reagents, b) decrease of their electric surface charge, and c) reduction of their sialic acid content, is shown to be regularly and, to all appearances, directly associated with weakness or absence of MN antigenic substance; it is not seen in comparable "variants" of the Ss antigens. This type of modified cell membrane is thus found in the presence of Mg, of weak forms of M or N (N2 with or without a positive direct antiglobulin test, or as produced by the gene complexes MS.Sta, Ms.Sta, MsMi.V or NsMi.V) and when antigen production at the MN locus is inhibited, completely or almost completely, by the "operator" genes En or Mk. Apparent exclusions of parentage due to some of the genes involved, particularly Mg, En and Mk, are presented. On the other hand, the NNSs genotypes of members of the 3 known families with an En(a-) propositus are discussed and a revised interpretation of them is given, based on renewed serological studies of the persons concerned. A genetical scheme of the MNSs system, comprising four "structural" loci-Ena MN, U Ss-and three "operators", is proposed.