A large interest of MIM/OMIM in chromosomal aberrations is in connection with gene mapping. Notable examples of identification of the chromosomal locus of Mendelian disorders through finding small interstitial deletions in sporadic cases include retinoblastoma (MIM +180200) on chromosome 13, Duchenne muscular dystrophy on Xp, and adenomatous polyposis coli on chromosome 5. In all three cases, it was the cytogenetic clue that led to isolation of the mutant gene. One of the earliest examples of deletion mapping was assignment of the ABO blood group—adenylate kinase—nail-patella syndrome cluster of gene loci to 9q34 by Ferguson-Smith et al.44 Aniridia (MIM #106210) was mapped to 11p13 by its occurrence alone or as part of the Wilms-aniridia-genitourinary-mental retardation syndrome (WAGR [MIM #194072]) in patients with deletions in that region of 11p.