A protein consisting of all or part of the amino acid sequences (known as the “domain”) of two or more proteins. Formed by fusing the two protein-encoding genes (which causes the ribosome to subsequently produce the fusion protein). This fusion is often done deliberately, either to put the expression of one of the (fused) genes under the control of the strong promoter for the first gene, or to allow the gene of interest (which is difficult to assay) to be more easily studied via substituting some of the (gene) protein with a more easily measured (assayed) function. For example, fusing a difficult-to-study gene with the β-galactosidase gene, the (protein) product of which can easily be measured (assayed) using chromatography.