Mcl-1 expressed by mast cells in vitro and maps of the Cpa3-Cre and Mcl-1^fl transgenes. (A) Mcl-1 is expressed by mast cells in vitro. BMCMCs (5 × 10⁵) from male wild-type C57BL/6J mice were subjected to electrophoresis and Western blotting using a polyclonal Ab against Mcl-1 (Rockland) and reprobed with anti-GAPDH Ab to show loading. Bands represent the long (35 kDa) and short (32 kDa) forms of Mcl-1. BMCMCs were cultured for 24 hours with 10% FCS alone (IL-3⁻) or FCS + IL-3 (IL-3⁺; 20% Wehi-3–conditioned medium). Expression of Mcl-1 decreased after 24 hours of culture in medium without added IL-3. Results are representative of those obtained in 6 independent experiments. (B) Map of the Cpa3-Cre transgene. A 780-bp fragment of the region just 5′ to the Cpa3 transcription start site was used to drive expression of Cre recombinase in vivo (Cpa3-Cre). These transgenic mice were bred with mice in which the first exon of Mcl-1⁴ has been flanked by loxP sites (triangles).