The average steady-state levels of XOR mRNA (▪), β-casein (♦), whey acidic protein (WAP; •) and α-lactalbumin (alpha-lac, ▴) in mammary tissue are shown for the indicated days post coitus. RNA levels were determined by RT-PCR using the primer pairs described in Methods and normalized to those of β-actin. The points show the mean ± s.e.m. for 3-4 animals. The error bars for some points lie within the symbols. The inset shows the change in the average steady-state levels of XOR in the mammary gland (closed bars) and liver (open bars) during pregnancy, lactation and involution, as determined by S1 nuclease protection. RNA values are averages of duplicates from different animals expressed as the amount of radioactivity (cpm) associated with protected bands after normalization to 25 μg of total RNA. Individual values were within 10 % of the averages. Parturition in our colony occurs between days 19 and 20 post coitus.