Distinction between progestin- and glucocorticoid-binding sites in mammary glands. Apparent lack of cytoplasmic progesterone receptors in lactating mammary glands.

The cytosol fraction of the lactating mammary glands of mice does not appear to contain detectable amounts of progesterone receptors. Mixing experiments indicate that the absence of receptors is not due to interference by other factors in the cytosol. However, in the cytosol of mammary glands, there is specific binding or progestins to certain low-affinity sites which have characteristics of specific glucocorticoid-binding sites.