HSB-2 is a cell line derived from a patient who had T-cell acute lymphoblastic leukemia (T-cell ALL) with a t(1;7)(p34;q34). We used a genomic probe from the T-cell receptor beta (TCR beta) locus (7q34) to identify DNA rearrangements in HSB-2. Two rearranged BglII DNA fragments were cloned, and one of these clones was shown to contain the translocation breakpoint on the derivative chromosome I [der(I)]. We used a probe derived from this clone to isolate an unrearranged phage clone encompassing the breakpoint at Ip34. The restriction map of this clone was compared to the published maps of known protooncogenes located at Ip32-34. By restriction mapping, Southern blot analysis, and DNA sequencing we showed that the translocation breakpoint on chromosome I is located within the first intron of the LCK gene. The LCK gene codes for p56lck, a member of the SRC family of cytoplasmic tyrosine protein kinases. There are two classes of LCK transcripts (type I and type II), each expressed from a distinct promoter, and each having a unique 5' untranslated region (UTR); the protein coding regions of the two classes are identical. The breakpoint in the t(1;7) separates the two LCK promoters and juxtaposes the constant region of the TCR beta locus with the proximal promoter and with the protein-coding region of the LCK gene on the der(I) chromosome.