Four-color fluorescence in situ hybridization on a Drosophila embryo. A late blastoderm stage (nuclear cycle 14) embryo was probed with four different RNA probes. Blue: sog labeled with DNP, followed by a rabbit anti–dinitrophenyl-KLH IgG antibody (Cat. No. A6430) detected with an Alexa Fluor® 647 chicken anti–rabbit IgG antibody (Cat. No. A21443). Green: ind labeled with biotin, followed by streptavidin HRP and Alexa Fluor® 350 tyramide (TSA Kit #27, Cat. No. T20937). Red: msh labeled with digoxigenin followed by sheep anti-digoxigenin antibody detected with an Alexa Fluor® 488 donkey anti–sheep IgG antibody (Cat. No. A11015). Yellow: sna labeled with fluorescein followed by mouse anti-fluorescein antibody detected with an Alexa Fluor® 555 goat anti–mouse IgG antibody (Cat. No. A21424). Image contributed by Dave Kosman and Ethan Bier, University of California, San Diego.