1. An in vitro method of screening transformed or transfected cells to identify those cells expressing simultaneously and/or sequentially at least two peptides or proteins of interest in predetermined ratios, including: (a) transforming or transfecting cells with at least two expression cassettes, wherein each expression cassette includes a gene encoding a peptide or a protein of interest linked at its 3′ end to an attenuated internal ribosome entry sequence (IRES) wherein the attenuated IRES is linked at its 3′ end to a fluorescent marker gene, such that the genes and their respective attenuated IRESs are in the same orientation, and wherein the detectable fluorescence of the products of the marker genes attached to each attenuated IRES are different; and wherein at least one of the attenuated IRESs is selected such that the fluorescent marker gene to which it is linked is expressed less than the peptide or protein of interest to which it is linked; (b) expressing the genes encoding the peptides or proteins of interest such that expression of the genes encoding the peptides or proteins of interest results in expression of the fluorescent marker genes; (c) identifying cells expressing at least two peptides or proteins of interest in predetermined ratios by detecting fluorescent signals from the products of the marker genes in the transformed or transfected cells using a fluorescence detection means.