Human Growth Hormone gene ( GH1 ) resides on chromosome 17q22-24 and it is expressed in somatotropic cells of the anterior pituitary gland. While there are multiple causes of GH Deficiency (GHD) a significant proportion have a genetic basis. The most severe Mendelian form of IGHD, called IGHD IA, has an autosomal recessive mode of inheritance. While affected individuals can have short lengths at birth and hypoglycemia in infancy, all develop severe dwarfism by six months of age. Although short stature, delayed growth velocity, and delayed skeletal maturation all occur with IGHD, none are specific for IGHD 1A. GH1 gene deletions, frameshifts, or nonsense mutations cause complete absence of GH in IGHD 1A. Thus IGHD 1A is best described as being complete GHD caused by severe loss of function GH1 gene mutations rather than being limited to only those having GH1 gene deletions. Interestingly, GH1 gene deletions are recurring mutations that can arise through unequal recombination in meiosis rather than by allele sharing through common descent. Individuals with IGHD 1A develop severe dwarfism in early infancy and often develop anti-GH antibodies after receiving exogenous GH. These antibodies can prevent the growth response expected from exogenous GH therapy. Individuals who are heterozygous for a GH1 gene deletion but whose other GH1 allele is not deleted and produces a non truncated product are usually immune tolerant.