The invention provides an agent that reduces the expression of G.alpha..sub.12 or G.alpha..sub.13 polypeptide, as well as an agent that enhances G protein G.alpha..sub.12 or G.alpha..sub.13 expression and/or activity. An agent of the invention may be used to decrease or increase G protein G.alpha..sub.12 or G.alpha..sub.13 expression and/or activity thereby to treat or prevent the onset of a disease or condition associated with G.alpha..sub.12 or G.alpha..sub.13 expression and/or activity. The invention also provides a method for screening for an anti-cancer or anti-angiogenesis agent, as well as an agent that promotes angiogenesis.