A Study of JNJ-66525433 in Healthy Participants and Participants With Ulcerative Colitis

Inclusion Criteria:

For Part 1 and Part 2, healthy volunteers

• Have a body mass index (BMI) between 18 and 30 kilogram per meter square (kg/m^2) (BMI = weight/height), inclusive, and a body weight of no less than 50 kilogram (kg)and maximum weight of 100 kg

• Have normal bowel movements, ranging from 1 to 3 times daily to every other day andhave normal consistencies according to the Bristol Stool Scale, ranging from 3 to 5at admission

For Part 3, participants with ulcerative colitis (UC)

• Have a clinical diagnosis of UC at least 3 months before screening

• Have moderately to severely active UC, defined as a Week 0 mayo score of 6 to 12,inclusive, based on central read of the video endoscopy performed during thescreening

• Have a greater than or equal to (>=) 2 endoscopy subscore of the Week 0 mayo scorebased on central read of the video endoscopy performed during the screening

Exclusion Criteria:

For Part 1 and Part 2, healthy volunteers

• History of liver or renal insufficiency, significant cardiac, vascular, pulmonary,gastrointestinal, endocrine, neurologic, hematologic, rheumatologic, psychiatric, ormetabolic disturbances

• Known allergies, hypersensitivity, or intolerance to antisense oligonucleotides,JNJ-66525433 or its excipients

For Part 3, participants with UC

• Has UC limited to the rectum only or to <20 centimeter (cm) of the colon evaluatedby endoscopy at screening

• Presence of a stoma

• Presence or history of a fistula at any time

COVID-related

• If a participant is excluded due to recent Coronavirus disease (COVID-19) relatedfeatures, the reason for screen failure should be documented in the case report formunder the exclusion criterion of having a condition for which participation wouldnot be in the participant's interest or could confound study assessments

• The field of COVID-related testing (for presence of, and immunity to, the severeacute respiratory syndrome coronavirus 2 [SARS-CoV-2] virus) is rapidly evolving.Additional testing may be performed as part of screening and/or during the study ifdeemed necessary by the investigator and in accordance with current regulations /guidance from authorities / standards of care