The aim of this study is to characterize the pharmacokinetics and safety/tolerability of AFQ056 in children with Fragile X Syndrome(FXS)

Inclusion Criteria:

• Genetically confirmed diagnosis of FXS
• At Screening and first baseline, vital signs, body weight and body mass index (BMI) must be age-specific within normal ranges.

Exclusion Criteria:

• Use of any other investigational drug within 30 days or 5 half-lives (whichever is longer) of the investigational drug prior to screening until end of study visit.
• History of hypersensitivity to AFQ056 or any mGluR antagonist.
• Female patients who are confirmed or suspected to be sexually active.
• History or presence of any clinically significant disease of any major system organ class, within the past 2 years prior to screening including but not limited to psychiatric, neurological, cardiovascular, endocrine, metabolic, renal, or gastrointestinal disorders (except for typical features of FXS).
• Smokers.
• Loss of ≥10% of total blood volume within 8 weeks (or less if required for this age group and/or by local regulation) prior to dosing or longer if required for this age group and/or by local regulation.
• Significant illness that did not completely resolve at least four weeks prior to the first baseline visit.
• Any abnormal laboratory values at screening or first baseline that are in the opinion of the investigator clinically significant and may jeopardize the safety of the study subject.
• Use of (or use within at least 5 half lives before dosing) concomitant medications that are strong/moderate inhibitors or inducers of CYP1A1/2, CYP2C9/19 or CYP3A4
• History or presence of Hepatitis B/C or HIV at screening