ARX and proximal spinal muscular atrophy: For example, the Survival of motor neuron 1 (SMN1) and Survival of motor neuron 2 (SMN2) genes (“dark” by 94% and 88%, respectively), both associated with spinal muscular atrophy (OMIM * 600354 and * 601627), the Aristaless-related homeobox (ARX) gene (“dark” by 12.8%) associated with a spectrum of X-linked developmental disorders including Partington syndrome (OMIM * 300382), and the T-box transcription factor 1 (TBX1) gene (“dark” by 10.6%) associated with the 22q11.2 deletion syndrome (OMIM * 602054), are all located within “dark” genomic regions [20].