Trypan blue (trade name MembraneBlue, VisionBlue) is a vital stain used to selectively color dead tissues or cells blue. Live cells or tissues with intact cell membranes are not colored. Since cells are very selective in the compounds that pass through the membrane, in a viable cell trypan blue is not absorbed; however, it traverses the membrane in a dead cell. Hence, dead cells are shown as a distinctive blue color under a microscope. Since live cells are excluded from staining, this staining method is also described as a dye exclusion method. This dye may be a cause of certain birth defects such as encephalocele. Trypan blue is commonly used in microscopy (for cell counting) and in laboratory mice for assessment of tissue viability. The method cannot distinguish between necrotic and apoptotic cells. Trypan blue is also used in ophthalmic cataract surgery to stain the anterior capsule in the presence of a mature cataract, to aid in visualization, before creating the continuous curvilinear capsulorhexis.