We have previously characterized a locus control region for the GH1 gene consisting of four DNase I hypersensitive sites (HS) located between 14.5 and 32 kb 5′ to the GH1 gene transcription start site. Sequence analysis of the region between the GH1 gene and its most proximal HS (HSI) revealed a perfect match to the B-lymphocyte-specific CD79b gene. Restriction mapping and hybridization analysis of YAC and cosmid clones confirmed the close linkage of the CD79b gene to the hGH gene cluster and facilitated the assembly of a 100-kb physical map linking the hGH locus, the CD79b gene, and the more distant muscle-specific sodium channel α-subunit (SCN4A) gene.