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PEG10 is a retroelement-derived Mart-family gene that is necessary for placentation and has been implicated in neurological disease. PEG10 resembles both retrotransposon and retroviral proteins and forms virus-like particles (VLPs) that can be purified using iodixanol ultracentrifugation. It is hypothesized that formation of VLPs is crucial to the biological roles of PEG10 in reproduction and neurological health. Here, we describe the regulation of PEG10 VLP formation and release in human cells with a role for the related Mart gene RTL8. RTL8 resembles a truncated form of PEG10 that shares homology with the N-terminal gag-like capsid domain. Alone, RTL8 is unable to form VLPs, but was incorporated into PEG10-derived particles. RTL8 co-expression decreased the abundance of PEG10 VLPs and increased intracellular levels of PEG10, suggesting a model where RTL8 inhibits PEG10 VLP formation or release. Consistent with this model, RTL8 bound to the N-terminal domain of PEG10 capsid, and modulation of RTL8 influenced PEG10-derived VLP abundance in naturally producing cells. RTL8 is broadly expressed in many of the same tissues as PEG10, including in human brain. Taken together, these results describe a novel antagonistic relationship between two human retroelement-derived genes and have implications for our understanding of PEG10 biology and disease. [ABSTRACT FROM AUTHOR]