The SCL/TAL-1 gene encodes a basic helix-loop-helix (bHLH) transcription factor essential for the development of all hemopoietic lineages and also acts as a T-cell oncogene. Four related genes have been described in mammals (LYL-1, TAL-2, NSCL1, and NSCL2), all of which exhibit a high degree of sequence similarity to SCL/TAL-1 in the bHLH domain and two of which (LYL-1 and TAL-2) have also been implicated in the pathogenesis of T-cell acute lymphoblastic leukemia. In this study we describe the identification and characterization of a pufferfish gene termed SLP-1, which represents a new member of this gene family. The genomic structure and sequence of SLP-1 suggests that it forms a subfamily with SCL/TAL-1 and LYL-1 and is most closely related to SCL/TAL-1. However, unlike SCL/TAL-1, SLP-1 is widely expressed. Sequence analysis of a whole cosmid containing SLP-1 shows that SLP-1 is flanked upstream by a zinc finger gene and a fork-head-domain gene and downstream by a heme-oxygenase and a RING finger gene.