Effects of AFQ056 on Language Learning in Young Children With Fragile X Syndrome (FXS)

• Age 32 months to 6 years inclusive at Screening (visit 1).

• Has an FMR1 full mutation.

• \*\*Note Presence of mosaicism is allowed

• DQ\<75 calculated from the Mullen Scales of Early Learning at time of screening.

• Parent or legal guardian is available and able to communicate well with the investigator, comply with study requirements and provide written informed consent.

• \*\*Note\*\*The Parent or legal guardian who will be signing consent form, should be the individual administering the language intervention

• English is the primary language spoken in the home and the subject's first language is English.

• Meet criteria indicating evidence of intentional communication based on parent interview via a communication eligibility screening tool.

• \*\*Note\*\* On the Eligibility Screening Tool - Communication, the child must have at time of screening:

⁃ Section 1: Answer of YES; the child uses at least 5 spoken words to label items on a daily basis.

‣ OR

⁃ Section 2: At least 3 YES answers to items 1-10 if child does not have at least 5 spoken words.

• Produces 3 or more intentional acts of communication on the structured portion of the Weighted Communication play sample at time of screening.

• \*\*Note: subjects are permitted to use augmentative communication devices throughout the study if the device is the subject's primary form of communication and the device has been prescribed for the subject by an SLP.

• Stable behavioral and other therapy regimen for 30 days prior to screening.

• \*\*Note: Patients will be allowed to continue their standard of care therapies throughout the trial but these will not be changed during the placebo lead in or placebo controlled portion of the trial, outside of the standard changes occurring from school schedules.

• Stable dosing of all concurrent psychotropic medications except stimulants for at least 60 days prior to screening. Due to the very short half-life of stimulants (specifically methylphenidate and amphetamine variants), a stable regimen of these medications is required for 2 weeks only.

⁃ Note\*\* Medications impacting GABA, glutamate and/or mGluR5 pathway receptors are exclusionary and not permitted during study participation. Additionally, stimulant regimens may include combinations of short- and long-acting forms and may be taken with different timing or dosing on different days of the week (e.g. Doses may be skipped on weekends or days off school and extra doses may be given some days for therapy sessions later in the day). The intent is to keep the doses and regimen being used at the time of screening consistent during the trial even if there is some variation in how the medication is taken on different days.Use of CBD oil or hemp based substances legal for sale over the internet are allowed provided that the dosing regimen has been consistent for at least 60 days prior to screening and will remain the same throughout the trial.