A fusion complementary DNA in the T cell line HSB-2 elucidates a provocative mechanism for the disruption of the putative hematopoietic transcription factor SCL . The fusion cDNA results from an interstitial deletion between a previously unknown locus, SIL ( SCL interrupting locus), and the 5′ untranslated region of SCL . Similar to 1;14 translocations, this deletion disrupts the SCL 5′ regulatory region. This event is probably mediated by V-(D)-J recombinase activity, although neither locus is an immunoglobulin or a T cell receptor. Two other T cell lines, CEM and RPMI 8402, have essentially identical deletions. Thus, in lymphocytes, growth-affecting genes other than immune receptors risk rearrangements.