Summary.A cDNA encoding trypsin inhibitor CMe from barley endosperm has been cloned and characterized. The longest open reading frame of the cloned cDNA codes for a typical signal peptide of 24 residues followed by a sequence which is identical to the known amino acid sequence of the inhibitor, except for an lie/Leu substitution at position 59. Southern blot analysis of wheat-barley addition lines has shown that chromosome 3H of barley carries the gene for CMe. This protein is present at less than 2%-3% of the wild-type amount in the mature endosperm of the mutant Ris0 1508 with respect to Bomi barley, from which it has been derived, and the corresponding steady state levéis of the CMe mRNA are about 1%. One or two copies of the CMe gene (synonym Itcl) per haploid genome have been estimated both in the wild type and in the mutant, and DNA restriction patterns are identical in both stocks, so neither a change in copy number ñor a major rearrangement of the structural gene account for the markedly decreased expression. The mutation at the lys 3a locus in Ris0 1508 has been previously mapped in chromosome 7 (synonym 5H). A single dose of the wild-type alíele at this locus (Lys 3a) restores the expression of gene CMe (alíele CMe-1) in chromosome 3H to normal levéis.