Behavioral abnormalities, ranging from autistic symptoms to anxiety and mood lability, are now recognized as major clinical issues in RTT. These clinical manifestations are particularly problematic in higher functioning individuals since they seem to substantially affect their quality of life. Moreover, novel drug treatments aiming at RTT's neurobiological mechanisms have the potential of targeting these abnormal behaviors. Despite the recent emphasis on abnormal behaviors, little is known about their characteristics, severity, and impact. This is due, in part, to the paucity of behavioral measures compatible with the cognitive and motor impairments of RTT. The investigators currently have only one instrument with limited validation for delineating the range of behavioral abnormalities in RTT (the parent-rated Rett Syndrome Behavioral Questionnaire (RSBQ)), which is a significant shortcoming to the implementation of treatment trials. Moreover, there has not been a systematic approach for defining behavioral abnormalities in RTT (e.g., definition of mood abnormalities in RTT). These deficiencies have led the investigators to propose the creation of a broad-based behavioral outcome measure for use in RTT at different ages. Due to the complexity of the clinical evolution of RTT and the relative limited knowledge on its adult period, the investigators will focus on children with RTT ages 3-18 years. The new measure will allow for standardized quantifications of behavioral outcomes in treatment trials and, eventually, in clinical practice. The investigators recognize that developing an instrument with strong psychometric properties is a major endeavor; therefore, the investigators conceived this project as a multi-stage process.

The first stage, conducted with support of Rettsyndrome.org, consisted of developing a prototype or first version of a behavioral questionnaire for parents of children with RTT (3-18 years), which the investigators have termed RettBe 1.0. The initial part of this study will test the psychometric properties (i.e., structure, content validity) of RettBe 1.0 with an initial cohort of 100 participants. This stage intends to transform RettBe 1.0 into a fully developed behavioral instrument within the framework of the Natural History Study (RTT5211). Thus, in Aim 1 the investigators will evaluate psychometrically RettBe 1.0 following, in part, previous studies including their examination of anxiety instruments and adaptation of the Anxiety, Depression and Mood Scale (ADAMS) for RTT, and their adaptations of the Aberrant Behavior Checklist-Community (ABC-C) for fragile X syndrome and Down syndrome. In Aim 1, they will also refine RettBe 1.0 by adding new "missing" items based on parental input or clinician (PIs of sites involved) feedback. The resulting instrument, RettBe 2.0 will be tested in Aim 2.

Testing of RettBe 2.0 will be carried out with a new (naïve) validation cohort of 300 subjects and two raters (preferentially both parents/caregivers, alternatively one teacher or therapist), to determine inter-rater reliability. One rater, preferentially a parent, will be asked to also complete three other behavioral measures (RSBQ, ADAMS, ABC-C) for comparisons. Scores for RettBe 2.0 will be analyzed in terms of psychometric properties, as performed for RettBe 1.0. However, in addition to structure (construct validity) and content validity, the investigators will also examine convergent and discriminant validity by correlating domain RettBe 2.0 scores with those of comparable and non-comparable domain scores of the RSBQ, ADAMS, and ABC-C, respectively.

Finally, Aim 3 will use data from the RTT5211 protocol to determine the effect of age on RettBe 2.0 scores and the clinical and functional significance of the measure. Specifically, the investigators will examine score distributions throughout the age range, with special emphasis on early childhood (3-8 years), late childhood (8-12 years), and adolescence (\>12 years). Clinical and functional relevance will be determined by correlations with instruments measuring multiple parameters of clinical severity, specifically, the CGI-S, Clinical Severity Scale, and Motor Behavior Assessment. Relevance to quality of life will be evaluated by correlations with the Child Health Questionnaire-Parent Form 50 (CHQ-PF50; child-oriented measure) and The Short Form (36) Health Survey (SF-36v2TM Health Survey; parent-oriented measure). All these instruments are components of the core RTT5211 protocol. Finally, prior to its release as RettBe 3.0 (if psychometric properties are good-excellent), the investigators will obtain input from a panel of clinicians (site PIs and their designated clinicians) about content validity and clinical impact.