To verify the mutational map, positions with FMN-binding sites were selected and tested by site-directed mutagenesis. In this selection, we selected sites that retained their function despite the change in the amino acid characteristics. The in vivo detection of fluorescence indicated that all the mutants retained fluorescence as expected by the mutational map (Figure 4(c), (i)). However, mutants L65F and N104S showed significant reduction in fluorescence. To quantitatively examine these mutants, we cloned them to an expression vector pET-28a(+), and the protein was purified using Ni-NTA resin for in vitro fluorescent assay (Figure 4(c), (ii)). The relative fluorescent unit (RFU) indicated a pattern similar to that of the in vivo fluorescent assay, except for L65F. L65F showed a very weak fluorescence in the in vivo assay; however, the purified L65F showed clear cyan color. In addition, the excitation peak of L65F was measured to be at 524 nm, showing a 27-nm shift from the wild type (Figure 4(c), (iii), (iv)). We predicted that these results arise from the dramatic amino acid change of leucine into a bulky aromatic amino acid, phenylalanine. First, this change may cause a steric effect on the stability of the FMN binding to L65F mutant and disturb the bound FMN to be released, causing the shift in emission peak close to the FMN emission peak, which is 530 nm. Second, this change may cause structural changes in the FMN binding pocket, changing the energy state of the molecule and thus the changing the emission wavelength