Mutations located within the gene encoding the homeobox transcription factor, ENGRAILED 2 (EN2), have now been identified as molecular markers associated with susceptibility for autism and related disorders. Thus, the present invention relates to compositions in the form of diagnostic kits, primers and target sequences, for use in methods for determining the predisposition, the onset or the presence of autism spectrum disorder in a mammal. Moreover, therapeutic methods for treating a person inflicted with, or predisposed to, an autism spectrum disorder based upon modulating the level or activity of EN2 are also provided.