A boy trisomic for the distal portion of 13q was dissected in detail and compared to 8 cases of complete trisomy 13 previously studied in our laboratory. The comparison shows that the partial trisomy 13q case did not correspond well to a muscle phenotype based on 6 variations common trisomy 13, but rather to a larger muscle phenotype that included variations less frequently observed in complete trisomy 13. Additional cases of partial trisomy 13 must be studied before these findings can be related to specific portions of chromosome 13.