This placebo-controlled, randomized, observer-blind, dose-ascending study will investigate the safety, tolerability, pharmacokinetics and pharmacodynamics of single and multiple doses of RO5271983 in healthy volunteers. Healthy volunteers will be randomized to receive either a single oral dose of RO5271983 or placebo in the SAD part or multiple oral doses of RO5271983 or placebo in the MAD part. The anticipated time on study treatment is approximately 14 weeks for the SAD part and up to 8 weeks for the MAD part.

Inclusion Criteria:

• Healthy volunteers, 18-65 years of age
• Body Mass Index (BMI) between 18 and 32 kg/m2, inclusive
• In the SAD, females subjects must be surgically sterile or post-menopausal for the past year; in the MAD females may be of child-bearing potential but must use 2 methods of highly effective contraception

Exclusion Criteria:

• A history of clinically relevant cardiovascular, gastrointestinal, hepatic, renal, endocrine, pulmonary, neurological, psychiatric, allergic or skin disease
• Clinical significant abnormalities in laboratory test results
• Symptoms of an infectious disease including upper respiratory tract infection within one month of study start or a history of recurrent infections
• Smokers of >5 cigarettes/day within 3 months prior to admission and unable to stop smoking during study