SCLtTA transactivator construct and expression of cre in SCLtTA x Tet-O-cre reporter mice. (A, upper portion) Structure of the murine SCL locus (coding exons shaded in black) indicating the position of the 3′ enhancer fragment contained in the SCLtTA construct. (A, lower portion) Transgenic construct consisting of the SV40 minimal promoter upstream of the rabbit β-globin intron 2, the tetracycline-dependent transactivator protein tTA, and a 5.5-kb BglII fragment of the enhancer of the murine SCL gene. (B) Cre mRNA expression under the control of the murine SCL enhancer in different tissues of the 3 independent founder lines (founders 2, 8, and 19), which demonstrated detectable cre expression after being crossed to the Tet-O-cre responder strain. Twenty micrograms per lane of RNA extracted from the following tissues were loaded as indicated: bone marrow (Bm), intestines (Int), lung (Lu), lymph node (Ln), spleen (Sp), and thymus (Th). RNA from the spleen of an MMTVtTA-Tet-O-cre double-transgenic mouse (+) and from the thymus of an SCLtTA single-transgenic mouse (-) served as positive and negative controls, respectively. The Northern blot for cre expression showed 2 bands, as described previously,¹⁶  using a 1.0-kb fragment of the Tet-O-cre transgenic construct³⁴  as a probe, as described in “Materials and methods.” The Northern blot was stripped and reprobed using a GAPDH probe.