Somatic cell hybridization, as an aid in the study of human map, has made large strides during the past several years. Rodent-human hybrids are readily obtained, and selective systems are available to select hybrid cells to the exclusion of parenteral cells. Various species-specific genetic markers can be studied in the hybrids, and correlation of the expression of these markers with each other and with specific human chromosomes retained allows synteny analysis and chromosome assignment. Methods to determine the relative order of genes and their regional localization are also available. Using these methods, more than 60 human genes have been assigned to 22 human chromosomes.