This multi-center, randomized, double-blind, placebo-controlled, parallel-group study will investigate the efficacy and safety of RO4917523 in adolescent and adult patients with fragile X syndrome. Patients will be randomized to receive oral doses of 0.5 mg or 1.5 mg of RO4917523, or matching placebo once daily. The anticipated time on study treatment is 12 weeks.

Inclusion Criteria:

• Adult and adolescent patients, 14-50 years of age
• Diagnosis of fragile X syndrome with a confirmed fragile X mental retardation 1 (FMR1) full mutation and qualifying scores on the Aberrant Behavior Checklist (ABC) and CGI-S
• Patients must agree to either remain completely abstinent or to use two effective contraceptive methods during and 3 weeks after the study

Exclusion Criteria:

• Previous treatment with another metabotropic glutamate (mGLU) receptor antagonist within 18 months or with RO4917523
• Participation in a clinical trial involving an investigational (unapproved) drug within 3 months or 5 times the half-life (whichever is longer) before start of this study
• Any uncontrolled, unstable clinically significant psychiatric condition other than fragile X syndrome
• History of suicidal behavior