As part of an ongoing investigation of the regulation of gene expression in B cell development, we have obtained a genomic DNA clone encoding the human J chain protein. The nucleotide sequence of exons encoding the mature protein defines a 137 amino acid primary sequence similar to that previously determined at the protein level. Probes from the gene have been used to analyze J chain expression in human cell lines corresponding to pre-B and B lymphocytes. J chain RNA was detected in two of six human pre-B cell lines and in 8 of 10 B cell lines expressing various Ig isotypes. The expression of the J chain gene is, thus, not tightly linked to IgM or IgA secretion. Our data do not, however, support the recent suggestion (7) that synthesis of J chain precedes that of mu chain in B lymphocyte differentiation. Because of the presence of nine candidate polyadenylation signals (AATAAA or AATTAAA) downstream of the C-terminal coding block of the J chain gene, the 3' end of the gene could not be determined from sequence data alone. To define the 3' end, J chain RNA from a human B lymphocyte line was used to protect an end-labelled DNA fragment from S1 nuclease digestion. The sequence 40 basepairs 5' of the functional polyadenylation site identified by these S1 experiments is homologous the same region of a previously reported mouse J chain complementary DNA clone.