**4.1. FlimPIA in cells**

To determine if FlimPIA is applicable in cellulo or in vivo, the FKBP-Donor and FRB-Acceptor were transiently expressed in cultured cells (**Figure 9**) [7]. The response was clearly observed in cells when rapamycin was added, and the luminescence intensity increased depending on the concentration of rapamycin.

However, the maximal S/B ratio was less than 2.5, and the detectable range of the concentration of rapamycin was rather narrow. Although the S/B ratio of FRET is often as low as that of FlimPIA in cells, PCA gives a high S/B ratio both in vitro and in cellulo.
