The Lck promoter was used to drive immature T cell-specific expression of the prokaryotic beta-recombinase gene. Five lines where created, two with high levels of expression in the thymus (lines 1 and 2). Line 1 contains 15 transgene insertions whereas line 2 contains three. Expression levels were high in the thymus, lower in spleen and lymph nodes, and absent in liver and brain. (J:137580)