Observational, noninterventional, 3-year study to examine the presence of detectable differences in neurocognitive, developmental, motor, neurophysiologic, and quality of life measures over time in patients with cyclin-dependent kinase-like 5 deficiency disorder.

This is an observational study to examine the presence of detectable differences in neurocognitive, developmental, motor, neurophysiologic, and quality of life measures over time in approximately up to 100 patients aged birth to 55 years and diagnosed with cyclin-dependent kinase-like 5 deficiency disorder. Operational performance across age groups and languages will be assessed throughout the study, and Baseline cohort characteristics will be assessed throughout enrollment.

Inclusion Criteria:

1. Documented diagnosis of Cyclin-dependent Kinase-like 5 (CDKL5) deficiency disorder with pathogenic or likely pathogenic of CDKL5 variants
2. Ages newborn to 55 years old
3. Parent/Caregiver who is willing and capable of providing written informed consent
4. Parent/caregiver lives with or has daily contact with study participant and able to provide consistent information across the study

Exclusion Criteria:

1. Any clinically significant neurocognitive deficit not attributable to Cyclin-dependent Kinase-like 5 (CDKL5) deficiency disorder or a secondary cause that may, in the opinion of the investigator, confound interpretation of study results (e.g., extreme prematurity, other genetic variants, unclassified CDKL5 variant, abnormal brain imaging, imaging and/or injury not otherwise attributable to CDD).
2. Any condition that, in the opinion of the investigator, would put the patient at undue risk or make it unsafe for the patient to participate
3. Any condition which would make the individual or the caregiver, in the opinion of the investigator, unsuitable for the study

This is a multi-center study to be conducted in multiple countries. The study will enroll patients diagnosed with Cyclin-dependent Kinase-like 5 (CDKL5) deficiency disorder, from birth to 55 years of age, at a participating site.