The invention relates to inhibition of ornithine aminotransferase (OAT) for suppression of tumor cells proliferation. More particularly, the invention relates to methods of treatment of proliferative disorders by the selective inhibition of OAT, and further provides the use of OAT inhibitors, specifically, 5-amino-1,3-hexadienyl-carboxylic acid (Gabaculine), and Gabaculine analogue 8, for compositions and methods for the treatment of proliferative disorders such as hepatocellular carcinoma. The invention further provides methods and kits for the diagnosis of a pathologic proliferative disorder in a mammalian subject, based on determining the level of OAT expressed in a biological sample obtained from a subject.