Germ-line stem cells, the in vitro counterpart of spermatogonial stem cells in the testis, can self-renew in vitro for more than two years and, when transplanted into the seminiferous tubules of an infertile male mouse, can establish donor-derived spermatogenesis to transmit the donor haplotype to progeny. Upon extended in vitro culture, a second cell type, called multipotent GS or multipotent adult GS cells, also appears from GS cells that can be expanded selectively under culture conditions used for embryonic stem cells. Unlike GS cells, mGS or maGS cells show multipotency and produce teratoma upon transplantation into the seminiferous tubules of the recipient testis. The mGS and maGS cells originate from the cultured GS cells themselves at a low frequency and are not some leftover of earlier type of germ cells. During this conversion, the androgenetic genomic imprinting in GS cells also changes to ES cell-like pattern in mGS or maGS cells. Recently, we showed that mouse maGS cells are epigenetically stable for DNA methylation at imprinted Igf2- H19 gene cluster during in vitro culture and differentiation but re-acquire GS cell-like growth and differentiation characteristics with altered DNA methylation pattern when they are re-cultured in the GS-like conditions. Thus, at any particular time point, in vitro cultured GS cells may contain some contaminating mGS or maGS cells which may produce teratoma instead of initiating spermatogenesis upon their transplantation into recipient testis. Consequently, a molecular marker that can distinguish GS cells from mGS or maGS cells would be of potential value in both clinical and experimental research settings. MicroRNA are a class of 20�?5 nucleotide-long noncoding endogenous RNAs that post-transcriptionally modulate the gene expression through canonical base pairing between the seed sequence of the miRNA and its complementary seed match sequence in the 39UTR of target mRNAs. Imprinted miRNAs represent a family of miRNA that are mono-allelically expressed in a parent-of-origin manner and act in trans, generally outside the genomic region from where they arise. Genes encoding the imprinted miRNAs are mainly clustered in two chromosomal domains in mouse although few single imprinted miRNA are also present at several genomic regions. Furthermore, almost all well-characterized imprinted genes clusters such as Igf2-H19, Peg10, Copg2, Rasgfr1, Gnas-Nespas, Kcnq1 and Igf2r-Air also encode one or more imprinted miRNAs whose expression is restricted in a parent-oforigin manner and is controlled by DNA methylation at imprinting control region of the respective gene cluster. These imprinted miRNAs show distinct temporal- and tissue-specific expression patterns in different GSK2118436 tissues, including ES cells, and control a wide range of developmental and physiological pathways, including stem cell pluripotency and differentiation.