Messenger RNA has been isolated from cells of the human myeloma line 266BL which synthesizes IgE of the myeloma ND. A fraction enriched in mRNA for the e heavy chain was copied into DNA and the DNA was cloned in Escherichia coli A chemically synthesized oligonucleotide probe, based on the experimentally determined sequence of the specific message, was used to screen colonies. The largest E chain cDNA cloned, 2.0 kiobases, was characterized by restriction endonuclease mapping and DNA sequence analysis. It appears to encode the complete amino acid sequence ofthe E chain, including a signal peptide at the NH2 terminus as well as untranslated sequences at the 5' and 3' ends of the mRNA. The missing part ofthe previously published amino acid sequence.of the ND E chain was determined from the DNA sequence.The medical importance of IgE, which differs from other immunoglobulins in possessing an E heavy chain, stems from its central role in type 1 immediate hypersensitivity. IgE binds with high affinity to specific receptors on blood basophils, and its association with antigen then triggers an allergic reaction by causing degranulation of the cells (1). Because of the minute concentrations of IgE in normal serum (0.1 Ag/ml, compared with 13 mg/ml for IgG), studies on the purified protein have largely been confined to the secreted products of IgE myelomas. The incidence of IgE myelomas is low, reflecting the corresponding plasma cell population; only three cases (2-4) have been reported so far.For the study ofhuman e gene expression and the molecular and cellular bases of the pathogenicity of IgE, we wished to obtain a cloned DNA sequence that encoded a functional E chain. The usual path to such a clone is by way of isolation and enzymatic copying of mRNA from cells that synthesize the corresponding protein, followed by cDNA cloning. The usual source of immunoglobulin mRNA is an appropriate myeloma or myeloma cell line. In view of the rare occurrence of human IgE myelomas and the fact that human myelomas, in contrast to those of mouse and rat, are notoriously difficult to establish in culture, it was fortunate that Nilsson et aL (5, 6) were able to establish a cell line from the ND myeloma. Its recent adaptation to suspension culture greatly facilitated our endeavors in isolating E chain mRNA. The previously determined sequence of the ND e chain (7) enabled us then to prepare a genetic probe for selecting cDNA clones.The cloned DNA has been used to complete the protein sequence by analysis of the DNA, to correct the earlier variable region sequence and a part ofthe constant region sequence, and to determine the sequence ofthe NH2-terminal signal peptide.This new information has allowed us to identify the D and J elements expressed in the 266BL cell line. We have determined the DNA sequence encoding the 5' and 3' untranslated regions of the mRNA.
