Editor—Holt-Oram syndrome (HOS) is a developmental disorder characterised by malformations of the radial ray of the forelimb and by congenital heart disease.1 The syndrome shows a marked variability in phenotype, with radial ray defects ranging from minor thumb abnormality through to severe reduction defect or phocomelia. The cardiac manifestations of HOS are similarly varied, and patients can present with a variety of structural heart abnormalities, atrial septal defects (ASDs) and ventricular septal defects (VSDs) being the most common, or conduction defects evident on ECG profiles. Previous studies have shown no correlation between the severity of a patient's cardiac and skeletal abnormalities.2 Intrafamilial variation can be wide.

HOS shows autosomal dominant inheritance and mutations in the T box transcription factor gene (TBX5) have been shown previously to be responsible for this disorder.3 4There is also evidence for genetic heterogeneity.5 The mechanism by which mutations in TBX5 cause a dominant phenotype is not understood at present, and it is anticipated that knowledge of the type of mutations causing HOS may shed light on this. Knowledge of a large number of mutations and the relation of a person's genotype to phenotype is also useful for genetic counselling. In the face of a growing demand for a molecular diagnostic test for HOS, it is also helpful to have a quantitative estimate of the ability of current methods to detect mutations inTBX5.

Twenty five cases with a clinical diagnosis of Holt-Oram syndrome have been tested for this study, bringing to 47 the total number of cases studied by us. Minimal diagnostic criteria were as described previously2: bilateral radial ray defect, plus either cardiac abnormality or family history of cardiac abnormality. Cases were referred by a variety of clinicians and underwent full clinical assessment including …