Using a v-yes DNA as the probe, Yamanashi et al. (1987) screened a human cDNA library made from placental RNA and derived DNA clones representing a novel genetic locus termed LYN. Nucleotide sequencing showed that LYN encodes a novel tyrosine kinase. Northern hybridization analysis showed that a 3.2-kb LYN mRNA was expressed in a variety of tissues of the human fetus. The pattern of expression was different from those of related genes such as YES.The degranulation response was dependent on a rise in intracellular calcium that was inhibited in Lyn-deficient mast cells but intact in Fyn-deficient cells. Degranulation proceeded in Lyn -/- cells due to increased activation and constitutive phosphorylation of the calcium-independent protein kinase C delta isoform (PRKCD).