In addition to the Mendelian mutations, several instances of heritable phenotypic variation have been reported. We have observed, in mice, a role for sperm RNAs in the induction of such stable phenotypic variation. When experimentally transferred by RNA microinjection into fertilized mouse eggs, the noncoding RNAs homologous in sequence to the target locus are efficient inducers of variation at the transcriptional level. Transmission of the phenotypic variation to progeny is highly efficient and independent of gender. Here, we have summarized these finding and how they relate to other reports of epigenetic variation.