LTK, ALK and Ros have been identified as receptor tyrosine kinases having sequence similarity to the insulin receptor subfamily of kinases. LTK, leukocyte tyrosine kinase, is expressed in murine B-lymphocyte precursors and has also been found in forebrain neurons. ALK, anaplastic lymphoma kinase, is normally highly expressed specifically in the nervous system. A truncated form containing the catalytic domian of ALK is expressed as the result of a translocation occuring in many non-Hodgkin’s lymphomas. The c-Ros gene was originally identified in mutant form as an oncogene. Ros is normally expressed in a small number of epithelial cell types and may play a role in epithelial development.