Abstract: A method for detecting an interaction between two tester proteins. In a cell incapable of activating a Ras protein, two nucleic acid sequences are expressed. One sequence encodes for a fusion protein comprising a mutant Ras protein incapable of localizing at the cell membrane and not requiring an exchange factor fused to one of the tester proteins. The other sequence encodes for the other tester protein fused to a plasma membrane localization domain. An interaction between the two fusion proteins leads to the expression of a functional Ras protein that is tested as an altered cell phenotype.