We produced double FP-tagged chimeras in order to assess whether the ECFP and EYFP reporters could be co-visualized within the same animal. The first series of chimeras were generated through the aggregation of tetraploid embryos with FP+ ES cells (or embryos) of the complementary color. The procedure is schematized in Fig. 3a. It is evident that the expression of the two FPs can easily be discerned in these double transgenic embryos where the diploid, embryo-proper compartment is ECFP+ and the tetraploid extraembryonic compartment is EYFP+ (Fig. 3b,3c,3d,3e,3f). Secondly we investigated reporter co-visualization in balanced adult chimeras comprised of both ECFP+ and EYFP+ compartments. These were generated through the aggregation of diploid embryos with diploid embryos (or ES cells) of the complementary color. The aggregated components of the chimera from which each organ originated is schematized in the top right of each panel in Fig. 4. These chimeric adult organs demonstrate that the ECFP+ and EYFP+ cells comprising these chimeras can easily be discerned (Fig. 4). It can therefore be envisaged that two different FP colors could be used in the construction of chimeras so as to tag both mutant and wild type lineages.