In an effort to understand the genetic bases of cancer and other diseases, candidate genes must be isolated and characterized. The human gene mapping project is participating in the construction of physical and linkage maps of the human genome. These maps then serve as a framework for characterizing and identifying novel sequences. Three primary methods are utilized for mapping genes. Polymerase chain reaction (PCR) typing of somatic cell hybrid DNAs allows assignment to specific chromosomes. Fluorescence in situ hybridization permits high resolution localization to metaphase and interphase cells. Genetic linkage analysis using reference families and PCR assays of highly polymorphic microsatellites enables the calculation of recombination distances between markers and disease phenotypes. Recently assigned genes include cytokines, neuron growth factors, RAF oncogenes, and lymphokine receptors. High resolution physical and genetic mapping of chromosome 3p has resulted in the identification of a homozygous deletion in small cell lung carcinoma, and has produced a candidate gene for Von Hippel-Lindau disease.