The relationship between HPRT deficiency and impaired DA neuron development has been partially clarified by recent studies that have demonstrated that HPRT deficiency leads to broad alterations of DA neuron-related transcription factors and aberrant neurite outgrowth and cellular morphology in mouse MN9D DA neuroblastoma and human NT2 embryonic carcinoma undergoing neuronal differentiation in vitro [13], [14]. This evidence concerns the gene HPRT1 and embryonal carcinoma.