Fig. 4. Rescue of the lactation-deficient phenotype after three successive post-partum periods (A,C,E) and their matching involution phases (B,D,F) in heterozygous prolactin receptor knockout mice that carry in addition the transgenic reporter constructs (WAP-Cre/Rosa-lacZ). Note the simultaneous increase in the number of parity-induced (X-Gal positive; blue) epithelial cells after each involution period and the reversion of the lactation-deficient phenotype and formation of normal secretory lobules after the third pregnancy (E). B, lower left, magnification of a selected area on the right to demonstrate the presence of a few X-Gal positive cells after the first reproductive cycle.