From these observations we conclude that the blue-shifted ECFP, and the red-shifted EYFP variants of wtGFP are both amenable to use in mice. Also since we have established lines of viable and fertile mice having widespread FP expression we have demonstrated that ECFP and EYFP represent developmentally neutral reporters. Since we were unable to obtain ES cells, embryos or mice with widespread expression of a RFP (DsRed1) we conclude that this reporter is not developmentally neutral or that its expression cannot be sustained at levels high enough for detection, and as such is not as versatile as other gene-based fluorescent reporters.