Promoting the discussion on the role of placenta in developmental programming. The discovery of elevated maternal plasma STC1 in pregnancy complications warrants further investigations of its potential as a biomarker. They can also be targeted to specific sub-cellular organelles and do not leak out of the cells, allowing long-time-course recording. Two Benzethonium Chloride structurally different classes of GECI can be recognized: a Fo��rster resonance energy transfer -based type that relies on the use of calcium binding element interposed between two fluorescent proteins and a second class where the calcium sensor uses a single FP. The latter class comprises sensors such as G-CaMP and Pericam which respond to calcium binding by changing their fluorescence intensity. In an effort to improve and expand the hue range of G-CaMP type sensors, which are based on circular permutated GFP, a recent study has published a colony-based screen for Ca2+-dependent 
fluorescent changes. This screen has produced a new set of sensors, called GECO, with different calcium dynamic range and fluorescent hues. CatchER has been developed for detecting high calcium in the endoplasmic reticulum and although it is based on a single FP, it differs from the other member of this class of sensors in the fact that the calcium binding site has been introduced into the eGFP itself, adjacent to the chromophore. The most widely 3,4,5-Trimethoxyphenylacetic acid employed Calcium FRET sensors are the Cameleon sensors. They comprise a fusion of the calmodulin protein and the calmodulin-binding domain of myosin light chain kinase M13 inserted between two fluorescent proteins such as CFP and YFP. Upon binding of calcium to calmodulin, the M13 chain binds to the calmodulin protein, bringing the two fluorophores into close proximity and allowing energy transfer to occur. However calmodulin is a ubiquitous signalling protein and may interfere with the expressed Cameleon sensors and at the same time the over-expressed sensors may also deregulate cell signalling. In order to bypass this issue, a different set of calcium FRET biosensors that employ Troponin C as the calcium-binding moiety have been generated. Troponin C is selectively expressed in skeletal muscle cells and therefore does not interfere with normal cellular processes when introduced in cell lines not derived from myocytes. In particular the TN-L15 sensor developed by Heim et al. consists of a chicken skeletal muscle Troponin C segment inserted between the fluorescent proteins DC11CFP and the improved yellow mutant of YFP called Citrine. 14 amino-acids of the Nterminus of the Troponin C fragment were removed in order to optimize the efficiency of the energy transfer. The binding of calcium to the binding sites induces a conformational change bringing the two fluorophores closer together, and allowing energy transfer to occur. The efficiency of the Fo��rster resonance energy transfer depends on the relative geometry between the two fluorophores as well as their spectral properties. The methods most widely applied to read-out FRET efficiency are intensity-based or fluorescence lifetime-based measurements. Spectral ratiometric read-outs are sensitive to spectral cross-talk, requiring additional calibration samples for quantitative measurements, and can be more severely impacted by photobleaching, optical scattering and the inner filter effect, which can be important when measuring signals in biological tissue. We therefore believe it is useful to characterise the performance of FRET biosensors using the fluorescence lifetime approach, which only requires measurements of the donor emission.