Inclusion Criteria:

• Entry and completion of Study M16-067. Completion includes the final endoscopy of Study M16-067. If the final endoscopy for Study M16-067 is missing during the COVID-19 pandemic due to local regulation prohibiting endoscopy, subjects may be allowed to enroll in Substudy 3 should they meet clinical response. 
• Achieved clinical response, defined as decrease in Adapted Mayo Score ≥ 2 points and ≥ 30% from Baseline, PLUS a decrease in RBS ≥ 1 or an absolute RBS ≤ 1 at the last visit of Study M16-067. For subjects with missing final endoscopy for Study M16-067 due to COVID-19 pandemic, clinical response is defined as a decrease in Partial Adapted Mayo Score ≥ 1 point and ≥ 30% from Baseline, PLUS a decrease in RBS ≥ 1 or an absolute RBS ≤ 1 at the last visit of Study M16-067.
• If female, subject must meet the criteria as stated in Contraception Recommendations.
• Subject must be able and willing to give written informed consent and to comply with the requirements of this study protocol, including self-administration or care-giver administration of SC injections. In Japan, if the subject is < 20 years old, a subject's parent or legal guardian must be willing to give written informed consent.

Exclusion Criteria:

• Subject is considered by the Investigator, for any reason, to be an unsuitable candidate for the study. Subjects should not be enrolled in Study M16-066 if high grade colonic dysplasia or colon cancer is discovered at the endoscopy performed at the final visit of Study M16-067. 
• Subject who has a known hypersensitivity to risankizumab or the excipients of any of the study drugs or the ingredients of CHO, OR had an AE during Study M16-067 that in the Investigator's judgment makes the subject unsuitable for this study.
• Confirmed positive urine pregnancy test at the Final Visit of Study M16-067.
• Subject is not in compliance with prior and concomitant medication requirements throughout Study M16-067.
• Subject with any active or chronic recurring infections based on the Investigator's assessment makes the subject an unsuitable candidate for the study. 
• Have a known history of lymphoproliferative disease, including lymphoma, or signs and symptoms suggestive of possible lymphoproliferative disease, such as lymphadenopathy and/or splenomegaly.