ATR and osteoporosis: While ATR is essential for embryonic development (Brown and Baltimore, 2000), mosaic deletion of ATR in adult mice (ATRmKO) via a tamoxifen-inducible form of Cre recombinase leads to the rapid appearance of a number of age-related phenotypes including hair graying, alopecia, kyphosis, osteoporosis, premature thymic involution, loss of spermatogenesis and increased tissue fibrosis (Ruzankina et al, 2007).