CLOCK-MS is a multi-site, phase IV clinical trial (sponsored by EMD Serono) designed to understand the mechanism of action of Cladribine (Mavenclad®) tablets by exploring the effect on CNS biomarkers relevant in the relapsing forms of MS. The study is designed to generate hypotheses regarding the impact and relevance of Cladribine tablet activity in the CNS by assessing the cerebrospinal fluid (CSF) levels of lymphocyte subsets, other immune cells, neuronal injury markers, and soluble immunological markers in study participants with relapsing forms of MS before and during treatment with Cladribine, and the association of these CSF markers with corresponding blood markers and with clinical outcomes. ClinicalTrials.gov Identifier: NCT03963375