CSH1 (chorionic somatomammotropin hormone 1) is a pregnancy-specific hormone produced exclusively by placental trophoblastic cells that plays critical roles in fetal growth, metabolism, and lactation stimulation 1. Unlike growth hormone, CSH1 activates only the prolactin receptor (PRLR) through zinc-induced dimerization, not the growth hormone receptor [UniProt]. CSH1 expression is regulated by a stress-responsive signaling cascade involving MAPK8/9-dependent upregulation of transcription factors EOMES and HAND1, as well as PRKAA1/2-dependent downregulation of ID2 2. CSH1 haploinsufficiency appears clinically relevant to growth regulation, with heterozygous deletions detected in Silver-Russell syndrome patients and isolated growth retardation cases at higher frequency than in controls 3. Placental CSH1 expression is significantly altered in gestational diabetes, with distinct expression signatures differentiating gestational diabetes subtypes A1 and A2 from type 2 diabetes and healthy pregnancies 4. Additionally, CSH1 has emerged as a Breg-related gene in bladder cancer, where CSH1 knockdown in B cells increases regulatory B cell phenotype and immunosuppressive function against CD8+ T cells 5. At the molecular level, CSH1 is dynamically expressed in syncytiotrophoblast nuclear subtypes and is downregulated following RYBP chr17 remodeler knockout 6.