This study will evaluate the safety, tolerability and efficacy of multiple doses of AFQ056 in patients with Fragile X Syndrome. The dose range will be 50 to 150 mg b.i.d. The primary read-out of efficacy is reduction in Aberrant-Behavior Checklist score.

Inclusion Criteria:

• Male, non-smoking patients between 18 and 35 years of age (both inclusive).
• Patients with fmr1 full mutation (> 200 CGG repeats)
• Patients with a Clinical Global Impression Severity Score (CGI-S) of > 4 (moderately ill)
• Patients with a score of >20 in the ABC-C scale (at screening)
• Patients with a mental age of ≥ 48 months as measured by the Stanford-Binet test

Exclusion Criteria:

• Patients with DSM-IV diagnosis of schizophrenia, history and/or presence of psychosis, confusional states and/or repeated hallucinations.
• Patients with a history of seizures in the past 5 years without any therapeutic treatment controlling the disorders.
• Patients under stable anti-convulsant therapies that experienced seizures in the 2 years prior to randomization
• Patients with ECG abnormalities, autonomic dysfunctions, bronchospastic diseases, drug or atopic allergy
• Any surgical or medical condition which might significantly alter the absorption, distribution, metabolism or excretion of drugs
• Patients using (or have used within four weeks before randomization) concomitant medications that are potent inhibitors of CYP3A4 (e.g., ketoconazole, ritonavir, etc.)

Other protocol-defined inclusion/exclusion criteria may apply.