GIN1 (Gypsy retrotransposon integrase 1) encodes a protein containing an integrase domain highly similar to those found in Gypsy/Ty3 retrotransposons 1. The protein shares structural similarity with retroviral integrases but surprisingly did not derive directly from retrotransposons. Instead, phylogenetic and structural analyses indicate that GIN1 and its paralog GIN2 evolved from novel animal-specific DNA transposons called GIN elements, which appeared early in animal evolution through co-option of retrotransposon integrase sequences 1. GIN1 is present across multiple vertebrates and shows homology in urochordates, suggesting an ancient evolutionary origin 1. The gene may function in controlling the expansion of GIN elements and Gypsy/Ty3 retrotransposons in chordates 1. In terms of disease relevance, GIN1 has been identified as a genetic marker associated with rheumatoid arthritis susceptibility, showing association with the disease regardless of anti-citrullinated peptide antibody status 2. However, the specific molecular mechanism by which GIN1 contributes to RA pathogenesis remains unclear from available evidence.