The biosensors consist of a single protein containing cyan fluorescent protein, an ABA receptor region, a protein region that naturally binds to the ABA-activated receptor, and yellow fluorescent protein. Cyan fluorescent protein is excited by blue-violet light. For the ABACUS bioprobes, binding to ABA causes the protein to fold up and brings the cyan and yellow fluorescent proteins into close proximity. This increases the efficiency of FRET (cyan arrow), and shifts the colour of fluorescence from more cyan to more yellow (Jones et al., 2014). The ABAleon bioprobes are designed the same way, but work differently in that binding to ABA actually shifts the fluorescence the other way: from more yellow to more cyan (Waadt et al., 2014).