SPY555-FastAct™ is a new and unique fluorescent live cell actin probe. It labels very dynamic actin filaments as well as the more static ones. SPY555-FastAct™ is a bright, fluorogenic, non toxic, cell permeable and highly specific live cell probe for fluorescence imaging of actin. It does not require any transfection and works just by adding the probe to the cells and perform imaging a couple of hours later.