Small RNAs in Mammalian Germline: Tiny for Immortal

Germ cells are the only immortal cells in a mammalian organism. Here, I review recent progress in the research on the role of small non-coding RNAs - namely microRNAs (miRNAs), endogenous siRNAs (endo-siRNAs), and piwi-interacting RNAs (piRNAs) - in the development of mammalian germ cells. Two key functions of small RNAs in germ cells are to globally regulate the germ cell developmental program and to keep selfish genetic elements under strict surveillance in order to maintain the germline immortality and to keep the species stable and eternal. I propose that many new members of small RNAs and even completely new types of small RNAs acting in the germline, especially in early primordial germ cells (PGCs) will be discovered in the near future.