Cerebral Cavernous Malformations (CCM/MIM 604214) are vascular malformations characterised by abnormally enlarged capillary cavities without intervening brain parenchyma. Clinical manifestations include seizures, cerebral haemorrhages and focal neurological deficits. They occur as a sporadic or autosomal dominant condition. Most often, sporadic cases have only one lesion and familial cases are characterised by a high frequency of multiple lesions. Three CCM loci were previously mapped on 7q (CCM1), 7p (CCM2) and 3q (CCM3) and CCM1 gene was identified as coding Krit1, a protein of unknown function, which was shown initially to interact in yeast two hybrid assays with Rap1A, a small ras GTPase and more recently to Icap1alpha, a modulator of beta1 integrin signal transduction. Herein, we screened KRIT1 gene in 121 unrelated, consecutively recruited, CCM probands having at least one affected relative and/or showing multiple lesions on cerebral MRI. Fifty-two of these probands (43%) were shown to carry a KRIT1 mutation. Forty-two distinct mutations were identified including six recurrent ones. Three-quarters of these mutations were located in the C-terminal half of the gene, mostly within exons 13, 15 and 17. All of them are predicted to lead to a premature stop codon. No missense mutation was identified. The only two nucleotide substitutions predicted to be missense mutations led in fact to an abnormal splicing and a premature stop codon. Altogether these data suggest that KRIT1 mRNA decay due to the presence of premature stop codons and Krit1 haploinsufficiency may be the underlying mechanism of CCM.