Tnk1 is a nonreceptor protein tyrosine kinase (NRPTK) and a new member of the Ack family of NRPTKs. Tnk1 is a ubiquitously expressed 47-kDa protein with autotyrosine kinase activity that is developmentally regulated during embryogenesis. Tnk1 is also upregulated following IL3 withdrawal from factor-dependent murine NSF/N1.H7 cells that undergo apoptosis, suggesting a role in growth inhibition. Data support a negative regulatory role for Tnk1 in regulating the Ras-Raf1-MAPK growth pathway by a mechanism that requires its autotyrosine kinase activity.