A ubiquitin promoter drives expression of the 'EKAREV' FRET biosensor for activated ERK. This comprises a central core composed of a ligand domain (the WW phosphopeptide-binding domain of the Homo sapiens PIM1 gene), a flexible linker and a sensor domain (an ERK substrate peptide), with the core sandwiched between two fluorescent proteins, YPet and ECFP, that act as FRET acceptor-donor pair. Upon ERK activation, the sensor domain is phosphorylated and can be interact with the ligand domain. This change in conformation results in a change in FRET efficiency from the donor to the acceptor. The sensor is tagged at the C-terminal end with a Tag:NES(HIV-1-Rev) nuclear export sequence (PMID:21976697).