They looked at 242 RA patients with respect to erosion number and size on high-resolution peripheral quantitative CT (HR-pQCT) scans of the metacarpophalangeal joints.20 They showed ACPA and RF to have an additive effect on erosion number and size, and interestingly, that RF influenced erosion size only in APCA-positive but not ACPA-negative patients. This evidence concerns the gene PRTN3 and rheumatoid arthritis.