CSHL1 (chorionic somatomammotropin hormone like 1) is a placenta-expressed member of the growth hormone/chorionic somatomammotropin (hGH/CSH) gene cluster at 17q22-24 1. This gene encodes a placental hormone with growth factor activity and growth hormone receptor binding capacity [GO annotations]. CSHL1 likely functions as a gestational hormone that compensates for the absence of other GH/CSH cluster members during intrauterine development [UniProt]. The gene operates through growth hormone receptor signaling and JAK-STAT pathways to regulate feto-maternal glucose metabolism and organ development [GO annotations]. CSHL1 expression is dynamically regulated throughout pregnancy in relation to birth weight outcomes. In large-for-gestational-age (LGA) newborns, CSHL1 expression is significantly increased compared to appropriate-for-gestational-age controls, whereas pregnancies with small-for-gestational-age (SGA) newborns show down-regulation of the entire hGH/CSH cluster 2. Circulating CSHL1 levels follow a temporally regulated pattern in uncomplicated pregnancies and are significantly disrupted in preeclampsia 3. Additionally, CSHL1 represents an abundant placental transcript 4 and may serve as a diagnostic biomarker for amyotrophic lateral sclerosis pathogenesis 5. Maternal tobacco smoke exposure alters CSHL1 expression in umbilical cord tissue, suggesting environmental influences on placental hormone regulation 6.