A Study to Test an Oral Medicine, Belumosudil, in Combination With Corticosteroids in Participants at Least 12 Years of Age With Newly Diagnosed Chronic Graft Versus Host Disease.

This is a parallel, Phase 3, two-arm study for the treatment of newly diagnosed moderate or severe chronic GVHD. The study duration for a participant includes up to 4 weeks for screening; a treatment period until clinically meaningful cGVHD progression (defined as progression requiring addition of new systemic treatment for cGVHD), relapse/recurrence of the underlying disease, participant starts new systemic treatment for cGVHD or experiences an unacceptable toxicity, at the request of the participants or the investigators, or until the end of study is reached, whichever comes first; at least 30 days follow-up of adverse events (AEs) after the last dose until resolution or stabilization, if applicable; and long-term follow-up until death or study close-out, whichever comes first.