Study Participants: - Mothers who are at minimum 18 years of age and their infants who are at least 36 weeks gestation age at birth and at least 3 weeks old by Study Day 0 - Multiple births may be allowed following discussion with the medical monitor. Note: in the opinion of the healthcare practitioner (HCP), there are no infant or maternal issues that would preclude the mother participant or the infant from participation - Lactation history is well established, with the mother exclusively breastfeeding her infant - Mother participant is receiving mirikizumab therapeutically for an approved diagnosis (having received at least 2 doses of mirikizumab SC maintenance therapy every 4 weeks prior to study Day 0) - Have a breastfed infant who can feed from a bottle with previously stored breast milk or formula

• Have a history of inadequate lactation, for multiparous participants who have previously breastfed - Have a significant previous or current history of comorbidities capable of significantly altering the absorption, metabolism, or elimination of drugs, or of constituting a risk when taking mirikizumab, or of interfering with the interpretation of data - Have any condition that fulfills any contraindication stated in the label. All warnings and precautions stated in the label need to be considered - Have a history of breast augmentation procedures judged to have a clinical impact on breast milk expression such as breast reduction surgery and some breast implants, or current evidence of acute or chronic conditions affecting breasts and potentially interfering with breast milk collection - Have received any live vaccine (that is, live attenuated) within less than 4 weeks or inactivated vaccine within less than 2 weeks before enrollment or intend to receive a live vaccine during the study or an inactivated vaccine 2 weeks postdose, the mother participant must be given the vaccine at an injection site remote from mirikizumab administration