Lymphocytes represent basic components of vertebrate adaptive immune systems, suggesting the utility of non-mammalian models to define the molecular basis of their development and differentiation. Our forward genetic screens in zebrafish for recessive mutations affecting early T cell development revealed several major genetic pathways. The identification of lineage-specific transcription factors and specific components of cytokine signaling and DNA replication/repair pathways known from studies of immuno-compromised mammals provided an evolutionary cross-validation of the screen design. Unexpectedly, however, certain pre-mRNA processing factor genes, including tnpo3, encoding a regulator of alternative splicing, were also found to play a specific role in early T cell development. In both zebrafish and mouse, TNPO3 deficiency impairs intrathymic T cell differentiation, illustrating evolutionarily conserved and cell type-specific functions of certain pre-mRNA processing factor. Overall design: Taking advantage of the apparent evolutionary conservation of lymphocyte-based immunity, we conducted genetic screens in zebrafish aimed at identifying novel regulators of T lymphocyte development. Apart from mutations in genes encoding lymphoid lineage-specific transcription factors, and components of cytokine signaling and DNA replication/repair pathways, mutations in genes encoding pre-mRNA processing factors were also found. To examine the molecular consequences, transcriptome analyses were conducted for three mutants, snapc3, lsm8, tnpo3.