ER expression in mammary glands treated with progestins for 2 months. For both (a) ER-α and (b) ER-β, a representative image of four to 10 histological slides/group is shown. The number of stained cells was quantified and expressed as a percentage of the number of stained cells among epithelial cells in 400× fields of every slide (bar: 80 μm). A minimum of 1,500 epithelial cells/slide were counted. The right panels show the quantification in control (filled columns), MPA-treated (dotted columns), and Pg-treated (empty columns) mice (mean ± standard error). ER-α was significantly lower in C57BL/6 mice (P < 0.001), and there was a significant decrease with MPA treatment in BALB/c mice. Regarding ER-β, there was a significant increase in progestin-treated BALB/c mice. In control groups, animals were killed at diestrus; animals were treated for 2 months. *P < 0.05, ***P < 0.001 control versus treated mice. ER, estrogen receptor; MPA, medroxyprogesterone acetate; Pg, progesterone.