We synthesized 2 CK‐13 peptides with citrullines at positions 451 and 456 (cCK13‐1: 444TSNASGR‐cit‐TSDV‐cit‐RP458), or at positions 451 and 457 (cCK13‐2: 444TSNASGR‐cit‐TSDVR‐cit‐P458), and mapped antibody responses to these peptides in a US cohort of 287 RA patients and 330 OA controls. This evidence concerns the gene KRT13 and rheumatoid arthritis.