Part 2: To assess the efficacy of combination therapy with GS-1427 and ustekinumab, compared with GS-1427 and ustekinumab monotherapies, in achieving clinical response at Week 12 conditions: Ulcerative Colitis studyType: INTERVENTIONAL phases: PHASE2 allocation: RANDOMIZED interventionModel: SEQUENTIAL primaryPurpose: TREATMENT masking: DOUBLE whoMasked: PARTICIPANT whoMasked: INVESTIGATOR count: 423 type: ESTIMATED name: GS-1427 name: Ustekinumab name: Placebo-to-match GS-1427 measure: Parts 1 and 2: Proportion of Participants Achieving Clinical Response at Week 12 measure: Part 2: Proportion of Participants Achieving Clinical Response at Week 24 measure: Parts 1 and 2: Incidence of Treatment-emergent Adverse Events (TEAEs), Serious Adverse Events (SAEs), or Deaths. measure: Parts 1 and 2: Incidence of Treatment-emergent Lab Abnormalities measure: Parts 1 and 2: Proportion of Participants Achieving Clinical Remission at Week 12 measure: Part 1: Proportion of Participants Achieving Clinical Remission at Week 52 measure: Part 2: Proportion of Participants Achieving Clinical Remission at Week 24 measure: Part 2: Proportion of Participants Achieving Partial Modified Mayo Score Remission at Week 52 measure: Parts 1 and 2: Proportion of Participants Achieving Histologic-endoscopic Mucosal Improvement at Week 12 measure: Part 2: Proportion of Participants Achieving Histologic-endoscopic Mucosal Improvement at Week 24 measure: Parts 1 and 2: Proportion of Participants Achieving Mucosal Healing at Week 12 measure: Part 2: Proportion of Participants Achieving Mucosal Healing at Week 24 measure: Parts 1 and 2: Proportion of Participants Achieving Endoscopic Improvement at Week 12 measure: Part 2: Proportion of Participants Achieving Endoscopic Improvement at Week 24 sex: ALL minimumAge: 18 Years maximumAge: 75 Years stdAges: ADULT stdAges: OLDER_ADULT facility: Om Research LLC status: RECRUITING city: Camarillo state: California zip: 93010 country: United States lat: 34.21639 lon: -119.0376 facility: VVCRD Research status: RECRUITING city: Garden Grove state: California zip: 92845 country: United States lat: 33.77391 lon: -117.94145 facility: Om Research LLC status: RECRUITING city: Lancaster state: California zip: 93534 country: United States lat: 34.69804 lon: -118.13674 facility: United Medical Doctors status: RECRUITING city: Murrieta state: California zip: 92563 country: United States lat: 33.55391 lon: -117.21392 facility: Encore Medical Research. LLC status: RECRUITING city: Hollywood state: Florida zip: 33021 country: United States lat: 26.0112 lon: -80.14949 facility: Encore Medical Research of Weston, LLC status: RECRUITING city: Weston state: Florida zip: 33331 country: United States lat: 26.10037 lon: -80.39977 facility: Clinical Research Institute of Michigan, LLC. status: RECRUITING city: Chesterfield state: Michigan zip: 48047 country: United States lat: 42.66281 lon: -82.84242 facility: Gastroenterology Associates of Western Michigan, PLC d.b.a. West Michigan Clinical Research Center. status: RECRUITING city: Wyoming state: Michigan zip: 49519 country: United States lat: 42.91336 lon: -85.70531 facility: Quality Medical Research status: RECRUITING city: Nashville state: Tennessee zip: 37211 country: United States lat: 36.16589 lon: -86.78444 facility: Southwest Clinical Trials status: RECRUITING city: Houston state: Texas zip: 77074 country: United States lat: 29.76328 lon: -95.36327 facility: Southern Star Research Institute, LLC. status: RECRUITING city: San Antonio state: Texas zip: 78229 country: United States lat: 29.42412 lon: -98.49363 facility: Tyler Research Institute, LLC. status: RECRUITING city: Tyler state: Texas zip: 75701 country: United States lat: 32.35126 lon: -95.30106 hasResults: False