SMTNL1 (smoothelin-like 1) is a muscle regulatory protein that functions as a key regulator of contractile properties in both skeletal and smooth muscle. 1 Mechanistically, unphosphorylated SMTNL1 inhibits myosin dephosphorylation, while phosphorylation at Ser-299 reduces this inhibitory activity, thereby modulating muscle contractility through interactions with contractile regulators including calmodulin, tropomyosin, and myosin phosphatase. 1 Additionally, phosphorylation at Ser301 by protein kinase A enables SMTNL1 to translocate to the nucleus where it functions as a transcriptional co-activator of progesterone receptor-B. 2 SMTNL1 demonstrates broad physiological relevance to metabolic and reproductive health. During pregnancy, SMTNL1 promotes skeletal muscle fiber-type switching from oxidative to glycolytic phenotypes through progesterone receptor interactions, 3 while also promoting endometrial epithelial cell differentiation and insulin sensitivity. 2 In vascular smooth muscle, SMTNL1 regulates myogenic reactivity and blood pressure through modulation of myosin phosphatase-targeting subunit 1 (MYPT1) expression. 4 SMTNL1 null mice exhibit impaired glucose tolerance, reduced reproductive fitness, and increased susceptibility to type 2 diabetes. 2 The protein also protects against hyperthyroidism-induced insulin resistance by suppressing glycolytic shifts in skeletal muscle. 5 These findings establish SMTNL1 as a critical therapeutic target for metabolic and reproductive disorders.