Small but sturdy: small RNAs in cellular memory and epigenetics.

Cells in multicellular organisms have distinct identities characterized by their profiles of expressed genes. Cell identities can be stable over a long time and through multiple cellular divisions but are also responsive to extracellular signals. Since the DNA sequence is identical in all cells, a "cellular memory" of expression profiles is achieved by what are defined as epigenetic mechanisms. Two major molecular principles--networks of transcription factors and maintenance of cis-chromatin modifications--have been implicated in maintaining cellular memory. Here we describe recent studies demonstrating that short noncoding RNAs can also provide molecular signals that define epigenetic states of cells. Small RNAs can act independently or cooperate with chromatin modifications to achieve long-lasting effects necessary for cellular memory and transgenerational inheritance.