DNA helicases (Hel) play a role in a number of processes involving DNA strand separation, including replication, repair, recombination, and transcription. Jarvis et al. (1996) identified a mouse gene encoding a novel putative helicase. They designated the gene LSH, for lymphoid-specific Hel, because Northern blot analysis revealed that it was expressed specifically in mouse early thymocytes. Geiman et al. (1998) determined that the mouse LSH, or HELLS (helicase lymphoid-specific), gene contains at least 18 exons and spans more than 26.6 kb. Using Northern blot analysis, these authors found that human HELLS is expressed in testis and thymus, two tissues that are highly active in DNA recombination. Geiman and Muegge (2000) demonstrated that LSH is not obligatory for normal lymphoid development but is essential for normal proliferation of peripheral T lymphocytes.