Mutations in the receptor expression enhancing protein 1 (REEP1) have recently been reported to cause autosomal dominant hereditary spastic paraplegia (HSP) type SPG31. In a large collaborative effort, we screened a sample of 535 unrelated HSP patients for "REEP1" mutations and copy number variations. We identified 13 novel and 2 known "REEP1" mutations in 16 familial and sporadic patients by direct sequencing analysis. Twelve out of 16 mutations were small insertions, deletions or splice site mutations. These changes would result in shifts of the open-reading-frame followed by premature termination of translation and haploinsufficiency. Interestingly, we identified two disease associated variations in the 3'-UTR of "REEP1" that fell into highly conserved micro RNA binding sites. Copy number variation analysis in a subset of 133 HSP index patients revealed a large duplication of "REEP1" that involved exons 2-7 in an Irish family. Clinically most SPG31 patients present with a pure spastic paraplegia; rare complicating features were restricted to symptoms or signs of peripheral nerve involvement. Interestingly, the distribution of age at onset suggested a bimodal pattern with the appearance of initial symptoms of disease either before the age of 20 years or after the age of 30 years. The overall mutation rate in our clinically heterogeneous sample was 3.0%; however, in the sub-sample of pure HSP "REEP1" mutations accounted for 8.2% of all patients. These results firmly establish "REEP1" as a relatively frequent autosomal dominant HSP gene for which genetic testing is warranted. We also establish haploinsufficiency as the main molecular genetic mechanism in SPG31, which should initiate and guide functional studies on "REEP1" with a focus on loss-of-function mechanisms. Our results should be valid as a reference for mutation frequency, spectrum of "REEP1" mutations, and clinical phenotypes associated with SPG31.