UASt regulatory sequences drive expression of 'CC3Ai', a fluorescent sensor for caspase activity (PMID:23857461). The sensor is initially synthesized as a linear protein composed of an N-terminal intein fragment, a C-terminal portion of the Cerulean fluorescent protein, a linker sequence that includes a DEVD caspase cleavage site, an N-terminal portion of the Cerulean fluorescent protein, a C-terminal intein fragment and a Tag:MYC tag. The two intein fragments result in self-splicing of the protein, producing a circular molecule that is not fluorescent (the Tag:MYC epitope is removed during the splicing process). In the presence of a caspase that can cleave the DEVD site, the protein is cleaved, reconstituting a fluorescent protein and allowing caspase activity (and thus activation of the apoptotic pathway) to be detected.