The authors describe the case of a child with craniofrontonasal syndrome (CFNS) (MIM 304110), the diagnostic process performed, the identification of the main clinical features in the proband (hypertelorism, facial asimmetry, bifid nasal tip, corpus callosum hypoplasia, broad thumb, curly and wiry hair), and the comparison with known data in literature. They also describe the detection, through gene sequencing of EFNB1, of responsible mutation and its correlation with the phenotypic variants. They explain the etiophatogenetic basis of the "unusual" inheritance pattern of CFNS: X-linked disease that occurs with greater severity in heterozygous females than hemizygous males. Finally, attention is placed on the need for careful genetic counseling for patients with CFNS, with special care in familial anamnesis taking. In the studied case, the presence of abnormalities of thumbs in the proband's mother and in two of her cousins, orientates principally toward a mutation of maternal origin or to a suspected somatic and germline mosaicism by creating a recurrence risk greater than general population. Because patients with CFNS reported in the literature are few, the AA consider that the observed case may help to improve understanding of the mechanisms of gene expression responsible for the syndrome, of its peculiar phenotypic manifestations and of its frequency in the population with known and easy to assign phenotypes, and possible mosaicisms that are difficult to detect.