Long interspersed elements 1 (LINE-1 or L1) are currently the only active form of autonomous retrotransposons that exist in the human genome. The 6 kb L1 element encodes its own proteins to mediate retrotransposition without the aid of components from other retrotransposons. Moreover, L1 not only can transpose its’ own element but L1 proteins are also used for retrotransposition by other retrotransposons in the human genome including short interspersed elements (SINEs), of which Alu elements are a subset. L1 elements are expressed in the pluripotent cells of the early embryo and are then repressed in most somatic cells. They are, however, expressed in neurons in the brain and can become de-repressed in some cancer cells. However, the consequence of activity in these cells is not fully understood. In this review, we discuss the approach of using proteomic analysis to understand the function of the most abundant of the L1-derived proteins, ORF1p, in human embryonic stem cells (hESCs) and how we can apply proteomic approaches to decipher the role of L1-derived proteins in neurons and cancer cells. We will also discuss other recent findings about ORF1p, and proteins and RNAs with which it associates in mammalian cells and discuss how these findings affect our view of how this protein may impact human biology.