ESCargo is a regulatable fluorescent secretory cargo protein that can be used to study the secretory pathway in vivo. It is a fusion protein comprised of a tripeptide endoplasmic reticulum signal (APV) that is recognized by SURF4 orthologs, a tripeptide N-glycosylation signal (NTT), DsRed-Express2 and the Tag:FKBP(C22V) reversible dimerization sequence. When targeted to the endoplasmic reticulum (via fusion with an appropriate signal sequence) and in the absence of 'SLF' (the synthetic ligand of the Tag:FKBP(C22V) sequence), the protein forms fluorescent aggregates in the ER. Upon addition of the SLF ligand, the aggregates dissolve into soluble tetramers that exit the ER (this process is dependent on the presence of a cargo receptor encoded by a SURF4 ortholog), resulting in a nearly synchronized cargo wave that can be used to study traffic through the secretory pathway (FBrf0247546).