A method for determining an interaction between a first protein and a second protein comprises the steps of: expressing in a cell or introducing into a cell a first fusion protein comprising the first protein, a multimerizable protein, and a fluorescent protein, and a second fusion protein comprising the second protein and a multimerizable protein; detecting a fluorescent focus formed by an association between the first fusion protein and the second fusion protein in the cell; and determining an interaction between the first protein and the second protein according to the detection of the fluorescent focus.