Three closely linked loci (IgG1, IgG2 and IgG3) were thought to be responsible for the Gm specificities. Van Loghem et al. (1970) presented evidence on the linkage relationship of immunoglobulin markers (gamma 1, 2, 3, Am).  A fourth IgG locus (gamma-G4) was identifiable in the cluster. Using a gamma-4 probe, Kirsch et al. (1982) assigned the IGH cluster to 14q32 by in situ hybridization. Bech-Hansen et al. (1983) found that an RFLP produced by BamHI is a marker for the heavy chain genes G2 and G4 and a gamma-pseudogene. Considerable linkage disequilibrium was found. Quantitative assessment of the degree of association between C-gamma RFLPs, Gm markers, and switch region RFLPs adjacent to C-mu and C-alpha-1 showed that the gamma-pseudogene is most tightly associated with G2, suggesting that it lies between A1 and G2.