In particular, in a large Swedish case–control study, dividing patients according to the ACPA (anti-citrullinated protein antibody) positivity, the RA risk linked to obesity appears different between genders; in fact, only the female gender was associated with the occurrence of ACPA-negative RA and there was no association between obesity and ACPA-positive RA among women, while an inverse association between BMI and ACPA-positive RA was seen in men (80). The gene discussed is PRTN3; the disease is obesity due to melanocortin 4 receptor deficiency.