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Objectives: To study the clinical character of every member of the family with two case on hypodontia by analysising common oral examination,pantomoagram radiographs of dentition,lateral skull X -ray film, the width of their full-mouth dental crowns and the family pedigree. Additionally, to study whether PAX9 is the pathogenic gene causing mandible lateral incisor missing in this family investigated.Methods: 1. Full-mouth teeth of every member of the family were copied, and the data about the tooth crown width of the full-mouth were obtained by electronic digital display caliper, and it was analysised whether the differences between the data and Chinese normal values.2.The features of bone and soft tissue about members with hypodontia in this family were analysised by lateral skull X-ray film, and were determined whether there are bone and soft tissue abnormalities after comparing with normal values.Results: 1.The width of tooth crowns onⅢ-1(proband)andⅡ-1 (proband,s father)are smaller than normal values. whileⅡ-1′s most of the teeth are normal, in addition to smaller mandibular incisor crown .2. The growth direction ofⅢ-1(proband)andⅡ-1(proband,s father)′s jaws show the level of growth. and both of them present convex facial type. The difference is thatⅢ-1(proband) present mandibular protrusion-based, andⅡ-1(proband,s father) is Maxillary protrusion Accompanied by inadequate facial height.3. It appeared same-sense mutation on the 717th nucleic acids of exon 3 in PAX9,and the 717th site of is C→T, the codon changing from CAC→CAT, the amino acid did,t changed.4.Ⅱ-2(proband,s mother) appeared missense mutation on the 718th nucleic acids of exon 3 in PAX9, and the 718th site is G→C.corresponding codon changing from GCG→CCG, and the amino acid changed from alanine (Ala) to proline (Pro).Conclusions: 1.Ⅲ-1(proband) andⅡ-1(proband,s father) have similar teeth and jaws abnormalities, and their characters show genetic tendency.2.Ⅲ-1(proband)andⅡ-1(proband,s father)′s 717th nucleic acid C site in exon 3 of PAX9 was turned into T, it appeared same-sense mutation.
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