Chorionic somatomammotropin hormone, also known as Choriomammotropin, Lactogen, Placental lactogen and CSH1, is a secreted protein which belongs to thesomatotropin / prolactin family. CSH1 is produced only during pregnancy and is involved in stimulating lactation, fetal growth and metabolism. Does not interact with GHR but only activates PRLR through zinc-induced dimerization. The CSH1 gene is member of the GH gene cluster on 17q, which consists of two growth hormone genes and three CSH genes. Genomic alterations in the GH cluster are well known, causing different phenotypes depending on the size of the deletion and the genes involved. The increased prevalence of hemizygosity of CSH1 in population in comparison to controls indicates a role for CSH1 haploinsufficiency in the etiology of growth retardation. Investigation of CSH1 deletions in further SRS and growth retarded patients will enable us to establish under which circumstances haploinsufficiency of CSH1 is likely to result in clinical changes.