C17orf1, a gene expressed in skeletal muscle and heart, was initially isolated from a fetal brain cDNA library and localized centromeric to and partially within the proximal CMT1A-REP element. A second gene, COX10, spans the distal CMT1A-REP element, and a duplicated exon of this gene is present in the proximal CMT1A-REP element. C17orf1 includes this duplicated COX10 exon within its sequence; however, the DNA strand opposite to that of the COX10 gene is utilized. We have determined the genomic organization of C17orf1 and found it to be oriented in the direction opposite to COX10. Analysis of the genomic structure of C17orf1 has revealed that it contains at least six exons and spans a length of at least 17 kb. All but one of the splice sites conform to the GT/AG rule, and in this case the splice acceptor site within intron 1 is GA instead of the expected AG. Sequencing and mapping analyses have shown that the centromeric boundary of the proximal CMT1A-REP element lies within intron 5. A 7-bp insertion, identified from genomic sequencing of cosmid clones and verified in the original cDNA clone and RT- PCR products, has extended the previously reported open reading frame from 591 to 756 bp. C17orf1 therefore encodes a 252-amino-acid protein.