**1. Introduction**

Huntington's disease (HD) is an autosomal dominant neurodegenerative disease caused by a Cytosine-Adenine-Guanine (CAG) repeat expansion of the *Huntingtin* gene (*HTT*). The disease typically presents in adulthood with an average age of onset of 40 (referred to as Adult Onset HD or AOHD), and is marked by a combination of motor, cognitive, and behavioral symptoms. Approximately 1–10% of patients with HD will experience onset of symptoms before age 21, which is categorized as Juvenile-Onset HD (JOHD) [1].

Evidence of behavioral disturbances in JOHD is primarily predicated on retrospective medical record analyses [2,3] and caregiver reports [4–6] with few attempts to systematically evaluate these changes prospectively. Behavioral problems and cognitive decline are often among the first symptoms to present in individuals with JOHD [6–9] and can emerge years before the onset of motor symptoms [5], a pattern that is parallel with that of AOHD. Behavioral issues reported in JOHD include violence, aggression, oppositional behavior, obsession, depression, anxiety, impulsivity, attention issues, psychosis, and substance abuse [1–4,8,10,11]. Family members and caregivers of individuals with JOHD have reported that behavioral symptoms are often more distressing and disruptive than motor symptoms [6].

The Kids-HD and Kids-JOHD study are parallel programs at the University of Iowa. The Kids-HD study enrolls children/young adults (ages 5–26 years old) who are at-risk for HD based on a parent or grandparent having been diagnosed with HD. These children are genotyped for research purposes only, and categorized into the Gene-Expanded (GE, CAG > 36) or Gene Non-Expanded (GNE, CAG < 35) group. Those that are GE will go on to develop AOHD later in life and those that are GNE will never develop HD. The Kids-JOHD study enrolls children/young adults (ages 5–26 years old) who are already symptomatic with JOHD and have had molecular confirmation of the gene expansion (motor diagnosis made prior to age 21). The GNE group makes an excellent comparison group for the JOHD sample given that although they did not inherit the gene, they are from a family in which a parent, and possibly other family members, are su ffering from HD, a family environment similar to the JOHD participants. The first aim of the current study was to establish group di fferences between JOHD and GNE participants based on informant ratings of behavior. The second aim was to examine a potential CAG repeat length e ffect in the JOHD participants. The third aim was to explore di fferences between self-reported and informant-reported executive function in participants over the age of 18.

#### **2. Materials and Methods**
