This is an open-label, multi-center, non-confirmatory study to assess the safety, efficacy, and cellular kinetics of YTB323 in approximately 28 participants with Relapsing Multiple Sclerosis (RMS) with breakthrough disease activity during previous treatment with a highly efficacious therapy (BD-HET). The study design utilizes an ascending single dose design consisting of 3 sentinel cohorts followed by an expansion cohort.