This study will prospectively enroll approximately 660 children, at least 18 months and less than 5 years of age, who have been referred to a pediatric developmental evaluation centers. Enrolled children will have blood drawn for RNA gene expression analysis and undergo a clinical evaluation to determine the presence or absence of a diagnosis of ASD.

The sequential co-primary objectives of this study are:

• To develop an algorithm to classify blood RNA gene expression patterns to maximize agreement between the classification and a clinical assessment of presence or absence of Autism Spectrum Disorders (ASD).
• To prospectively assess the clinical sensitivity and specificity of the blood RNA gene expression classification algorithm (the SDX-002 test) in children referred to a developmental evaluation clinic for a possible developmental disorder.

Inclusion Criteria:

1. Referred to a developmental evaluation centers for evaluation of a possible developmental disorder, other than isolated motor problems.
2. Age >= 18 months and < 5 years.
3. Parent/legal guardian has been informed about the study and has signed an informed consent form.

Exclusion Criteria:

1. Prior reliable diagnosis of Autism Spectrum Disorder (i.e. prior evaluation by a multi-disciplinary team has already reliably established Autism Spectrum Disorder diagnosis).
2. Unable or unwilling to complete study procedures

Children 18 months to < 5 years referred to a developmental evaluation centers for evaluation of a possible developmental disorder.