Etrolizumab, a monoclonal antibody to the β7 integrin, inhibits both α4β7:mucosal addressin cell adhesion molecule 1 (MAdCAM-1)–mediated lymphocyte trafficking to the gut mucosa and αEβ7:E-cadherin–mediated lymphocyte retention. In patients with ulcerative colitis (UC) from the Phase 2 EUCALYPTUS trial (NCT01336465), treatment with etrolizumab resulted in a reduction in serum levels of soluble (s) MAdCAM-1 compared with placebo; this reduction in sMAdCAM-1 paralleled a reduction in free β7 receptors in the peripheral blood, with complete receptor occupancy (RO) observed at day 5 after the first dose in both dose cohorts (100 mg and 300 mg).