Huntington disease: a case study describing the complexities and nuances of predictive testing of monozygotic twins.

When a candidate for predictive testing for the Huntington disease gene is a monozygotic twin, confidentiality of the co-twin's diagnosis and autonomy of participation are among the critical genetic counseling issues. Predictive testing can proceed when twins voluntarily and simultaneously request counseling and evaluation in an HD testing program. This case describes a young man referred for predictive testing to an HD testing site on the East Coast of the United States. Family history revealed a twin brother of unknown zygosity who resided on the West Coast of the United States. The genetic counselors on opposite coasts collaborated to provide genetic counseling and evaluation for voluntary, informed predictive testing of the twins, protecting their rights while observing national protocol guidelines