These diseases may arise from: (1) GH-releasing hormone receptor (GHRH-R) defects [15]; (2) GH gene deletion (isolated GH deficiency, IGHD) [16]; (3) GH receptor (GH-R) gene deficiency (Laron syndrome) [17]; and (4) IGF1 gene defects [5,18,19]. Here, GHR is linked to isolated congenital growth hormone deficiency.