Fig. 1. Physical map and exon organization of the fast/developmental myosin heavy chain genes. (A) The position, orientation and size of the MYH genes that compose the fast/developmental cluster. (B) The exon organization of each of the MYH genes. The position and exon organization of the fast/developmental MYH genes are based on analysis of the MYH region of human chromosome 17 using GeneQuest and the known cDNA sequences of the rat embryonic MYH gene (Strehler et al., 1986) and the rabbit MYH13 gene (Briggs and Schachat, 2000) to define exon boundaries. Only the positions of the coding exons (3-40 for the embryonic and MYH13 gene and 3-41 for the other members of the cluster) are presented. The predicted coding sequences exhibited greater than 99% identity with the cDNA sequences reported by Weiss et al. (1999b). Regions of homologous organization (exon—intron size and spacing) are indicated by distinctive colors. These structural homologies primarily reflect the order of gene duplication, although some, such as the two conserved patterns in the 5'-region of the MYH13 gene, are more probably the result of gene conversion events. This is readily evident for the segment from the beginning of exon 4 to the end of exon 5, which is identical to the corresponding sequence in the adjacent perinatal MYH gene.