Huntington disease--another chapter rewritten.

To those of us who began life when humans had 48 chromosomes and who began working in genetics when the (by then 46) chromosomes had no bands and chromosome 4 could not reliably be distinguished from chromosome 5, the mere ability to diagnose and correlate the clinical phenotypes of genetic disorders with their molecular genotypes is a source of continuing astonishment and pleasure. Indeed, molecular genetic analysis of neurogenetic disorders such as Huntington disease (HD) has provided a steady stream of challenges and surprises to all who believe the genetic principles that they were taught about these disorders. The paper by Rubinsztein et al. in this issue of the journal highlights yet another surprise, which was adumbrated even in the initial paper announcing the discovery of the HD gene: incomplete penetrance of HD gene mutations.