Trypan blue is a diazo dye that is widely used as a vital stain to selectively color dead tissues or cells blue. Due to its highly negatively charge nature, it is excluded from the intact cell membrane of viable cells. However, it can traverse the membrane in a dead cell which has a compromised membrane integrity. Once inside cell, trypan blue binds to intracellular proteins and render the cells a blue color. Trypan blue is commonly used in fluorescence microscopy and flow cytometry for viability assessment.