Heavy chain disease (HCD) is a naturally occurring lymphoproliferative disease in which variant monoclonal Ig heavy (H) chain fragments are found in serum or urine. Alexander et al. (1982) showed that the gene for the gamma-3 chain had undergone extensive NH(2)-terminal deletion. Cases of HCD involving immunoglobulins of the alpha and mu classes have also been described. Huck et al. (1986) cited several studies as yielding the following information on intervals between the several constant region genes of the heavy chains on chromosome 14: (qter) 5-prime--CM--4.5 kb--CD--CG3--26 kb--CG1--19 kb--CEP1--13 kb--CA1--CGP--CG2--18 kb--CG4--23 kb--CE--10 kb--CA2--3-prime (centromere). Huck et al. (1986) sequenced the CG3 gene from a member of the Tunisian family known to be homozygous for the G3m(b) allotypes.