ATR and isolated growth hormone deficiency type IA: Primordial dwarfism, a human disorder that involves an extreme reduction in growth and body size, is associated with loss-of-function mutations in genes coding several centrosomal proteins, including core components involved in centrosome duplication (e.g., PCNT, CPAP, and CEP152), the pre-replicative complex (e.g., ORC1, ORC4, ORC6, CDT1, and CDC6), and the DNA damage checkpoint (e.g., ATR and MCPH1)41.