4-Amino-1-[(2R,3R,4S,5R)-4-ethynyl-3,4-dihydroxy-5-(hydroxymethyl)oxolan-2-yl]pyrimidin-2-one, commonly known as 3'-β-C-ethynylcytidine or ECyd, is a synthetic nucleoside analogue. Its structure features a cytosine base attached to a modified ribose sugar ring where the 3'-carbon is substituted with an ethynyl group (-C≡CH). This modification is key to its biological activity. This compound is a potent cytidine deaminase-resistant nucleoside analogue with significant antiviral and antitumor properties. It functions as a ribonucleotide reductase (RNR) inhibitor, interfering with DNA synthesis and repair. Research has primarily focused on its potential as an anticancer agent, where it demonstrates activity against various solid tumors. Its mechanism involves intracellular phosphorylation to the active triphosphate form, which is incorporated into RNA, leading to chain termination and inhibition of protein synthesis. While its development as a therapeutic agent has been explored in clinical trials for cancer, its core identity remains that of a modified nucleoside with targeted biochemical activity. It serves as a valuable tool compound in biochemical and pharmacological research related to nucleotide metabolism and nucleic acid synthesis inhibition.