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Propionic acidemia is an inborn error of organic acid metabolism caused by deficiency of propionyl-CoA carboxylase (PCC: E. C. 6. 4. 1. 3.). We have detected three types of mutation in the same exon of the coding sequence of beta-subunit of PCC (beta PCC) from two ethnic background (Caucasians and Japanese): an insertion/deletion which replaces 14 nucleotides with 12 unrelated nucleotides results in the elimination of an Msp I site; a 3-bp inframe deletion results in loss of one of two consecutive isoleucine codons immediately preceding the same Msp I site; the C----T transition results a in loss of the same Msp I site. The insertion/deletion and the C----T transition show high allele frequency in Caucasians (0.32) and in Japanese (0.3), respectively. These results reveal the possibility of the independent origin of the mutation in the two ethnic backgrounds and suggest a key role of this exon in the structure and catalytic function of the beta-subunit of PCC.