Alternative splicing factor/splicing factor 2 (ASF/SF2) is the prototype of a family of nuclear proteins highly conserved throughout metazoa, the SR (serine/arginine) proteins. Based largely on in vitro studies, SR proteins have been suggested to play important roles in constitutive and alternative splicing of pre-mRNAs. Here we describe the development of a genetic system employing the chicken B-cell line DT40 to study the function of ASF/SF2 in vivo. The high level of homologous recombination and rapid growth rate of these cells allowed us to show first that ASF/SF2 is an essential gene, and then to perform targeted disruption of both ASF/SF2 alleles, by creating a cell line in which the only source of ASF/SF2 is a human cDNA controlled by a tetracycline (tet)-repressible promoter. We show that addition of tet to these cells results in rapid depletion of ASF/SF2, concomitant accumulation of incompletely processed pre-mRNA, and subsequent cell death. The tet-induced lethality could be rescued by plasmids expressing wild-type ASF/SF2, but not several mutant derivatives, or other SR proteins. Heterozygous cell lines overexpressing human ASF/SF2 displayed significant reductions of endogenous ASF/SF2 mRNA, suggesting that ASF/SF2 mRNA levels are controlled by an autoregulatory loop. This system provides a novel method for genetic analysis of factors that function in basic processes in vertebrate cells.