Early studies have shown that the mRNA molecules coding for mouse immunoglobulin(1g) light(L)-chains direct the cell-free synthesis of precursors somewhat larger than the mature protein, as estimated from electrophoretic mobility in sodium-dodecyl-sulfate(SDS)-polyacrylamide gels.'-' Some understanding of the functions of the precursors may be afforded by determination of their structure, i.e., the position (NH,-or COOH-terminal end), precise size, and primary structure of the extra peptide segment. This was first done in 1973, when the precursor of a mouse Ig L-chain, programmed by mRNA in vitro, was subjected to radioactive sequence analysis.' This study established two features common for all precursor molecules: in the precursor an extra peptide segment (about 20 residues long) precedes the NHz-terminus of the mature protein; this extra piece is enriched with leucine residues, thus indicating the marked hydrophobicity of the extra piece. Subsequently it was found that many other secretory proteins and integral membrane proteins were initially synthesized as precursor molecules having similar short-lived NH2-terminal extra pieces (see references 5 , 6, and this volume).The Ig molecule is composed of two L-chains and two heavy (H)-chains. The L-and H-chains comprise a very heterogeneous population of proteins, and they have unique structural features that make the study of their precursors of special interest. In the mature Ig chain the variable-region (V-region, the 110 NHderminal residues) exhibits sequence variability that is responsible for antibody diversity and specificity; the constant-region (C-region, the COOH-terminal portion of the protein) of H and L chains have a distinct sequence. We have isolated from myeloma tumors of mouse and rat the mRNA molecules that direct the cell-free synthesis of precursors, in which extra pieces (17-29 residues long) precede the NH,-termini of both the V-region (precursors of intact L-or H-chains) and Cx-region (precursor of the retype C-region fragment). The complete sequence of the NH,-terminal extra pieces of 10 Ig precursors were determined (FIGURE 1). The primary structures of these extra pieces and results of other experiments bear on problems of protein secretion in general and interaction of the precursor with cell membranes (endoplasmic reticulum and plasma membrane), as well providing new information concerning the organization and controlled expression of Ig genes.The Ig mRNAs were isolated from mouse or rat myeloma tumors that synthesize discrete Ig molecules. Tumors and their corresponding Ig chains are designated as M-321, MPC-11, IR-102, etc. The mRNAs were prepared from myeloma polysomes specifically precipitated by antibodies' directed to L-or H-chain.',' The Annals New York Academy of Sciences mRNAs were translated in the wheat germ cell-free system to provide precursor molecules that were subjected to radioactive amino acid sequence analy~es. ',~ SEQUENCE VARIABILITY AT THE NH,-TERMINAL EXTRA PIECE OF IMMUNOGLOBULIN PRECURSORS; THE VARIABLE-REGION GE...