To elucidate the potential mode of action of MAP-based cytolytic fusion proteins in living cells, colocalization assayswere performed using EGFR⁺ A549 cells transfected with a SNAP-tagged tubulin DNA construct. Expressed SNAP-tagged tubulin molecules were labeled with SNAP-Cell TMR-Star (green). EGF-MAP_f151 was used as a representative for MAP-based CFPs. MAP_f151 was detected using mouse–anti-human Tau and goat–anti-mouse Alexa Fluor 647 antibodies (red). DAPI was used to counterstain the nucleus (blue). Using confocal fluorescence microscopy and a tubulin polymerization assay, it could be shown that MAP colocalized with and stabilized microtubules. For more details, see article by Hristodorov and colleagues on page 2194.