With v-yes DNA as the probe, a human cDNA library made from placental RNA was screened under relaxed conditions, and DNA clones derived from a novel genetic locus, termed lyn, were obtained. Nucleotide sequencing revealed that lyn could encode a novel tyrosine kinase that was very similar to mouse T-lymphocyte-specific tyrosine kinase p56lck and the v-yes protein as well as to the gene products of v-fgr and v-src. Northern hybridization analysis revealed that a 3.2-kilobase lyn mRNA was expressed in a variety of tissues of the human fetus. The pattern of lyn mRNA expression was different from those of related genes, such as yes and syn. Hybridization analysis of DNA from sorted chromosomes showed that the lyn gene is located on human chromosome 8 q13-qter.