We report a 38-year-old female patient affected with anti-citrullinated protein antibody (ACPA)-positive rheumatoid arthritis (RA) who developed mild hemolytic anemia (Hb = 10.5 vs. >12 gr/dL), indolent oral ulceration, ANA (1:1280, homogeneous pattern), and anti-dsDNA antibody positivity following 8 months of therapy with an adalimumab biosimilar (GP2017). The gene discussed is GSTM1; the disease is rheumatoid arthritis.