Considering the facts that (1) the SCL locus is highly conserved in vertebrates [45], (2) there are observations showing that the murine SCL exon 4 has promoter activity in the context of SCL 3′ stem cell enhancer [46], and (3) a truncated SCL transcript initiated from exon 4 can be detected in some of the human T cell leukemia cell lines [28,29], we speculate that a scl-β equivalent may exist in higher vertebrate species. The gene discussed is TAL1; the disease is T-cell leukemia.