RT Journal Article SR Electronic T1 Familial predisposition to recurrent mutations causing Huntington's disease: genetic risk to sibs of sporadic cases. JF Journal of Medical Genetics JO J Med Genet FD BMJ Publishing Group Ltd SP 987 OP 990 DO 10.1136/jmg.30.12.987 VO 30 IS 12 A1 Y P Goldberg A1 S E Andrew A1 J Theilmann A1 B Kremer A1 F Squitieri A1 H Telenius A1 J D Brown A1 M R Hayden YR 1993 UL http://jmg.bmj.com/content/30/12/987.abstract AB Huntington's disease (HD) is associated with expansion of a CAG repeat in a new gene. We have recently defined a premutation in a paternal allele of 30 to 38 CAG repeats in the HD gene which is greater than that seen in the general population (< 30 repeats) but below the range seen in patients with HD (> 38). These intermediate alleles are unstable during transmission through the germline and in sporadic cases expand to the full mutation associated with the clinical phenotype of HD. Here we have analysed three new mutation families where, in each, the proband and at least one sib have CAG sizes in the HD range. In one of these families, two sibs with expanded CAG repeats are both clinically affected with HD, thus presenting a pseudorecessive pattern of inheritance. In all three families the parental intermediate allele has expanded in more than one offspring, thus showing a previously unrecognised risk of inheriting HD to sibs of sporadic cases of HD.