The present invention provides inhibitors of mucin-type O-linked glycosylation, and in particular inhibitors of polypeptide N-acetyl-.alpha.-galactosaminyltransferases; as well as compositions comprising the inhibitors. The present invention further provides methods of identifying inhibitors of polypeptide N-acetyl-.alpha.-galactosaminyltransferases. The inhibitors are useful in various applications, including research applications, and treatment methods.