Genetic studies on Drosophila Abl and, more recently, on mouse c-Abl and c-Src indicate that the functions of these non-receptor tyrosine kinases may duplicate activities of other molecules within signal transduction pathways. In Drosophila, second-site mutations have been recovered that disrupt the redundant functions so that the Abl tyrosine kinase is essential to the formation of axonal connections in the embryonic central nervous system and for attachment of embryonic muscles to the body wall. Molecular isolation and analysis of the genes identified by these second-site mutations should define the molecular basis for the genetic redundancy.