Single-point mutations in VL can disrupt Ig assembly and secretion completely, as exemplified by the FS62 (Phe62 to Ser) mutation in the λ1 chain (Dul and Argon, 1990), the GR15 (Gly15 to Arg) mutation in the λ2 chain (Wu et al., 1983), and the YH87 (Tyr87 to His) mutation in the κ chain, in MOPC 21 myeloma cells (Dul et al., 1992). The gene discussed is MLC1; the disease is plasma cell myeloma.