We describe the case of an adult patient affected by multiple exostoses, severe 
mental retardation, epilepsy and facial dysmorphisms with a deletion of ∼2.3 Mb 
on chromosome 11p11.21, correlated to Potocki-Shaffer syndrome (PSS). PSS is a 
rare contiguous gene deletion syndrome, mainly characterized by multiple 
exostoses and bilateral parietal foramina. Mental retardation and craniofacial 
dysmorphisms have often been reported, too. Although the patient showed many 
signs of PSS since early childhood, the diagnosis was suggested only when we 
examined her at adult age. This case highlights how frequently rare diseases 
remain undiagnosed till adulthood and is an excellent example of the need for a 
timely and correct diagnosis.