Ca2+-stimulated ribonuclease. A new marker enzyme of differentiated rat mammary tissues

A unique ribonuclease, active only in the presence of Ca2+, was present in lactating mammary gland and milk of the rat. This enzyme was absent from virgin-rat mammary gland and non-mammary tissues of lactating rats. The presence of moderate activity in differentiated mammary tumours, together with an increase in activity in normal tissue parelleling development of mammary function, identify this enzyme as a marker of mammary differentiation in the rat.