SLC22A4 and rheumatoid arthritis: However, the identification of OCTN1 as an RA susceptibility gene could not be verified in the Canadian (Newman et al. 2005), UK (Barton et al. 2005), or Spanish (Martinez et al. 2006; Orozco et al. 2006) population and could not be reproduced by another group working on the Japanese population (Kuwahara et al. 2005).