Similarly to POU5F1, LIN28 is a candidate marker of fGSCs as it is found in both stem cells and the germline. It is a gene of ancient origin, with homologues in many different species including Drosophila, mice and humans (Moss and Tang, 2003). It encodes for a RNA-binding protein initially discovered to be involved in the timing of embryonic development of C. elegans (Ambros, 2000). It was then discovered to be widely expressed in the developing embryos of other animals, but had more limited expression in differentiated adult cells (Moss and Tang, 2003). Subsequent studies in human ESCs (hESCs) demonstrated that LIN28 expression was downregulated as hESCs differentiated and suggested, therefore, that LIN28 could be used as a marker of stemness (Richards et al., 2004). As with POU5F1, both human (Yu et al., 2007) and bovine (Nong et al., 2015) embryonic fibroblasts could be reprogrammed to iPSCs by LIN28.

The role of LIN28 within the germline has been described with changing temporal and spatial expression over the course of the first and second trimesters in fetal human ovaries (Childs et al., 2012). Its expression is restricted to PGCs and is highest in the first trimester, with all PGCs expressing the protein. In the second trimester, only immature germ cells around the periphery of the ovary continue to express LIN28, with no expression detected in the more centrally located mature germ cells. Furthermore, LIN28-positive germ cells co-expressed POU5F1 but never demonstrated expression of the meiotic entry marker, SYCP3, indicating it is an early PGC marker (Childs et al., 2012). It has been suggested that LIN28 performs its pluripotency role by inhibiting the transcriptional processing of a microRNA (pri-let-7 miRNA) which is important in cell differentiation (Viswanathan et al., 2008). The similarities in temporal expression of LIN28 and its miRNA target across gestations in human fetal ovaries further supports that their functions are connected (Childs et al., 2012).