The ovarian hormones oestrogen and progesterone increase breast cancer risk but the cellular mechanisms are unclear. Here it is shown that the size of the mammary stem cell pool in mice is regulated by steroid hormone signalling, although these cells lack the receptors for oestrogen and progesterone. The augmented pool could lead to clonal expansion of a mutated cell, possibly accounting for the increased incidence of breast cancer associated with pregnancy.

The generation of induced pluripotent stem cells (iPSCs) from patients with defined genetic disorders promises to help the basic understanding of complex diseases and the development of therapeutics. Here iPSCs have been generated from patients with LEOPARD syndrome, a developmental disorder with pleiomorphic effects on several tissues and organs. The iPSCs are characterized and the phenotype of cardiomyocytes derived from these cells is investigated.

The association of microRNAs with Argonaute proteins (AGOs) yields complexes regulating gene expression. Although bacterial and archaeal miRNAs show no sequence preference at their 5? ends, eukaryotic miRNAs tend to have a 5? U or A. Here the structure of the human AGO2 MID domain complexed with ribonucleotide monophosphates is solved, revealing specific interaction of UMP and AMP with a loop that discriminates against CMP or GMP, and explaining the observed preference.