The present invention concerns a method of identifying a cell colony which expresses a soluble variant of a target protein which method comprises (a) subjecting the cell colony to conditions which are capable of causing lysis thereof; (b) filtering the lysate through a filter having pores which allow only soluble proteins to pass through the filter; and (c) detecting target protein which has passed through the filter where the target is not detected an the basis of its own enzymatic activity. The invention further covers the identification of a cell colony expressing a soluble variant of a membrane protein using steps (a) and (b) of the method above. A kit for use in the methods is also covered by the invention.