The h-prune protein is a member of the DHH protein superfamily, and its overexpression in breast, colorectal and gastric cancers correlates with depth of invasion and degree of lymph-node metastasis. Taken together with the observation that h-prune is highly expressed in metastatic breast cancer, this suggests that h-prune can be used as a marker for the identification of subsets of cancer patients with highly aggressive tumours. H-prune possesses a phosphodiesterase (cAMP-PDE) activity, and inhibition of PDE activity with dipyridamole suppresses cell motility. H-prune interacts with nm23-H1, GSK-3beta and gelsolin. Although a correlation between an h-prune PDE activity and cellular motility has been shown, GSK-3beta does not affect the PDE activity of h-prune. Inhibition of the interactions between h-prune and GSK-3beta and nm23-H1 additively suppresses the migration of colon cancer and breast cancer cells, thus suggesting that h-prune regulates cell motility by two different means of action: through its PDE activity and in its interactions with protein partners. Therefore, the identification of highly specific inhibitors of h-prune should be useful in the development of drugs to treat cancer metastasis.