A randomized, double-blind, clinical trial to assess the safety and efficacy of two doses of oral cladribine versus placebo in subjects who had a first clinical demyelinating event (clinically isolated syndrome). Subjects in either the cladribine or placebo group may also enter treatment periods with open-label interferon-beta or open-label cladribine depending upon the disease status. The primary objective of this study is to evaluate the effect of two dosage regimens of oral cladribine versus placebo on the time to conversion to multiple sclerosis (MS) (from randomization) according to the Poser criteria in subjects with first clinical demyelinating event at high risk of converting to MS.