SLC22A4 and Crohn disease: One recent example for which genetic hitchhiking has been proposed involves allele 503F of the ergothionene transporter OCTN1. Allele 503F correlates with an increased risk for Crohn's disease, but causal proof and a mechanism by which OCTN1 contributes to gastrointestinal autoimmunity in Crohn's disease has been elusive.