Long Interspersed Element 1 (LINE-1 or L1) is an autonomously mobile retrotransposon found throughout mammalian genomes. As a “selfish” genetic element, L1 must create new copies of itself in cells that will transmit their genetic information to the next generation. This process, termed retrotransposition, is mutagenic and can lead to cellular dysfunction and genetic disease. Therefore, our cells have evolved numerous mechanisms to impede L1 retrotransposition. Previously we demonstrated that L1 can retrotranspose in early mouse primordial germ cells (PGCs) which are specified early during mammalian embryonic development and ultimately give rise to all germ cells of an adult organism. However, little is known about the dynamics of L1 activity and its regulation during PGC specification, and early mouse PGCs in vivo are challenging to study. Here, we are developing a system to investigate regulation of L1 expression and retrotransposition during the specification of primordial germ cell-like cells (PGCLCs) from mouse embryonic stem cells (mESCs) in vitro. Our work will shed light on the unexplored interplay between mobile DNA activity and genomic defenses in this critical developmental niche.