An Efficacy and Safety Study of Palovarotene for the Treatment of MO

Key Inclusion Criteria: * Written, signed, and dated informed subject/parent consent and age-appropriate assent (performed according to local regulations). * A clinical diagnosis of MO with disease-causing exostosin 1 or 2 gene mutations. * Male or female from 2 to 14 years of age. * Female subjects must be premenarchal at screening. * A bone age at screening of 14 years or less. * Symptomatic MO, defined as five or more clinically evident osteochondromas and a new or enlarged osteochondroma that occurred in the preceding 12 months, five or more clinically evident osteochondromas and the presence of a painful osteochondroma, a skeletal deformity, a joint limitation, or prior surgery for a MO-related complication. * The ability to undergo whole body MRI with or without sedation/general anesthesia. * Use of two effective methods of birth control during treatment, and for 1 month after treatment discontinuation, unless committed to true abstinence from heterosexual sex. Sexually active females of child-bearing potential must also agree to start effective methods of birth control at screening. Key Exclusion Criteria: * Weight under 10 kg. * Other syndromic conditions such as Langer-Giedion or Potocki-Shaffer. * Any subject with neurologic signs suggestive of spinal cord impingement. * Concomitant medications that are strong inhibitors or inducers of cytochrome P450 3A4 activity. * Amylase or lipase \>2 times the above the upper limit of normal (\>2×ULN) or with a history of chronic pancreatitis. * Elevated aspartate aminotransferase or alanine aminotransferase above 2.5×ULN. * Any surgical implant that is contraindicated for MRI.