A central problem in cytogenetics is mapping the location of each gene. This involves determining their linear order and the physical distance and frequency of meiotic recombination between all components of each chromosome, especially the genes. The rate of progress in mapping the human chromosomes has increased rapidly. As recently as 1990, only about 1 % of our genes had been mapped, many of them not very accurately. In contrast, a recent paper describes a physical map of over 30,000 genes, estimated to be between one-third and one-half of the total human complement (Deloukas et al., 1998). It is no longer possible to maintain an up-to-date catalog in book form. The twelfth edition of Mendelian Inheritance in Man (McKusick, 1997) has mapping information on more than 4000 genes of known function. It is now accessible as an on-line database (http://www.ncbi.nlm.nih.gov/omim) known as OMIM. This is the only form in which the rapidly burgeoning body of information can be kept current.