The analysis of somatic CAG triplet variation in lymphoblastoid cell lines from subjects carrying alleles of intermediate length (IA33CAG and IA34CAG) in Huntington disease (HD) gene disclosed instability in the DNA of the person, from whom a new expansion mutation of 45 triplets originated. The triplet size increased after about 30 passages in cell cultures in lymphoblasts with the IA34 genotype. Lymphoblasts may provide an appropriate model for studying repeat instability in subjects with poly(CAG) repeat disorders. HD shows somatic, in addition to germ-line instability, highlighting the propensity to somatic CAG variation in human cells even with repeat numbers underthe expandededge. Factors potentially cis acting with the mutation, other than those reported in this study (CCG polymorphic stretch, the deletion of the glutamic acid residue at position 2642 and the 4-codon segment between CAG and CCG polymorphisms), should be searched for and analyzed.