DiI, DiO, DiD and DiR dyes are a family of lipophilic fluorescent dyes which stain membranes and other lipophilic molecules. Their fluorescence highly increases when incorporated into membranes or bound to lipophilic molecules. Once applied onto cells, they diffuse laterally within the membranes to stain the entire cells. Their fluorescence is significantly increased upon membrane incorporation. This family of dyes fluoresces at distinct colors: DiI (orange), DiO (green), DiD (red) and DiR (far red); so they are adapted to multicolor imaging and flow cytometric of living cells. DiO and DiI can be used with standard FITC and TRITC filters respectively. DiI is frequently used due to its very low cell toxicity. It is also used to label lipoproteins such as LDL and HDL.