Translin, a ubiquitous RNA/DNA-binding protein that forms a hetero-octamer together with Translin-associated factor X (TRAX), possesses endoribonuclease activity and plays a physiological role in restricting the size and differentiation of mesenchymal precursor cells. However, the precise role of Translin in epithelial cells remains unclear. Here, we show evidence that Translin restricts the growth of pubertal mammary epithelial cells. The mammary epithelia of Translin-null females exhibited retarded growth before puberty, but highly enhanced growth and DNA synthesis with increased ramification after the onset of puberty. Primary cultures of Translin-null mammary epithelial cells showed augmented DNA synthesis in a ligand-independent and ligand-enhanced manner. Translin-null ovariectomized mice implanted with slow-release estrogen pellets showed enhanced length and ramification of the mammary glands. Mammary epithelial growth was also observed in ovariectomized Translin-null mice implanted with placebo pellets. Luciferase reporter assays using embryonic fibroblasts from Translin-null mice showed unaltered estrogen receptor a function. These results indicate that Translin plays a physiological role in restricting intrinsic growth, beyond mesenchymal cells, of pubertal mammary epithelial cells. (C) 2019 Elsevier Inc. All rights reserved.