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 (TCRB) locus (7q34) t o identify DNA rearrangements in HSB-2. Two rearranged Bglll DNA fragments were cloned, and one of these clones was shown t o contain the translocation breakpoint on the derivative chromosome I [der(l)]. We used a probe derived from this clone t o isolate an unrearranged phage clone encompassing the breakpoint at lp34. The restriction map of this clone was compared t o the published maps of known protooncogenes located at I p32-34. 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 p56ICk, 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( I ;7) separates the two LCK promoters and juxtaposes the constant region of the TCRB locus with the proximal promoter and with the protein-coding region of the LCK gene on the der( I) chromosome.