Previously we described joining of DNA in the 9 T cell receptor gene to DNA of an uncharacterized locus in at (7; 9)(q34; q34. 3) chromosomal translocation from a case of human T lymphoblastic leukemia (T-ALL). We now show that the locus on chromosome 9 contains a gene highly homologous to the Drosophila gene Notch. Transcripts of the human gene, for which we propose the name TAN-f, and its murine counterpart are present in many normal human fetal and adult mouse tissues, but are most abundant in lymphoid tissues. In t (7; 9)(q34; q34. 3) translocations from three cases of T-ALL, the breakpoints occur within 100 bp of an intron in TAN-7, resulting in truncation of TAN-l transcripts. These observations suggest that TAN-7 may be important for normal lymphocyte function and that alteration of TAN-7 may play a role in the pathogenesis of some T cell neoplasms.