Two alpha heavy chain disease proteins with different genomic deletions demonstrate that nonexpressed alpha heavy chain genes contain methylated bases.

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Abstract
Two independently arising .alpha. H chain mutants synthesize H chains with CH1 deletions of approximately equal extent. Both were isolated from H chain-producing variants of the mouse myeloma W3129 and demonstrate that it is possible to arrive at the H chain disease phenotype by the pathway H + L .fwdarw. H .fwdarw. .DELTA.H. Analysis of genomic DNA by digestion with restriction endonucleases followed by molecular hybridization showed that 1 mutant (.DELTA.37) had a deletion of .apprx. 0.2 kilobase and the 2nd mutant (.DELTA.15) had a deletion of .apprx. 0.5 kilobase. Mouse myeloma cells contain several .alpha. chain alleles but only 1 is expressed; the presence of the deletion in .DELTA.37 and .DELTA.15 made it possible to identify the restriction fragments from the expressed allele. Analysis of the fragments produced after cleavage with an isoschizomeric pair of restriction enzymes Map I and Hpa II, indicated that, in the W3129 cell line and its variants, the unexpressed .alpha. alleles contain methylated bases. The influence of methylation on gene expression remains to be elucidated.