SMAD4 and cancer: That said, GSK3 inhibition has been proposed as a potential therapeutic option in specific forms of cancer [96], [130], [131], GSK3 activity is reported to reduce the production of at least two tumour suppressors (Smad4 and PHLPP1 in Table 1) and there has been at least one GSK3 inhibitor taken to clinical trial for anti-tumourogenic potential (Table 2).