Generation of IL-3–deficient mice. (A) Structure of IL-3 targeting vector and disrupted IL-3 gene. The 600-bp probe indicated identifies the 6.5-kb wild-type and 9.5-kb targeted fragments afterHindIII digestion as shown. X, Xba I; H,HindIII; B, BamHI; R, EcoRT. (B) Genotype of mutant animals. Tail DNA was digested with HindIII and probed as above. Molecular sizes are indicated on the left (in kilobases).