The objective of this project is to explore the potential of functional near-infrared spectroscopy (fNIRS) as innovative functional biomarker for clinical trial readiness in Creatine Deficiency Syndromes (CDS), a group of rare neurodevelopmental disorders. Among these disorders, effective treatments are lacking for two. The limited availability of objective and quantitative biomarkers to monitor brain function poses challenges to advancing therapeutic research. With gene therapy trials on the horizon, the need for precise measurement to evaluate treatment efficacy is pressing. This project seeks to address this gap by assessing the prognostic reliability of both resting and task-evoked fNIRS. Arousal of participants will be also assessed through the measure of spontaneous heart rate (HR) fluctuations. The primary objectives of this pilot study are: 1. to determine the feasibility of fNIRS in individuals with CDS; 2. to collect pilot data on individuals with CDS to determine the patterns of cerebral oxygen consumption as measured by fNIRS; 3. to compare cerebral oxygen consumption changes at rest and from visual/auditory tasks in affected individuals versus age-appropriate healthy volunteers. The secondary objectives of this study are: 1. to correlate cerebral oxygen consumption changes from visual/auditory task in affected individuals to other measures of disease state (e.g., neuropsychological assessment, disease- specific severity rating scales); 2. to examine test-retest reliability of our fNIRS measures in both affected individuals and healthy controls.