Lipids that exist as fats, namely the oily hydrophobic lipids, have an affinity for the sudan dyes. Oil Red O is an intense red stain for lipids. In order to penetrate the fats the dye must be dissolved in organic solvents, the solvent must be sufficiently dilute (aqueous) to avoid extracting the lipids into the solution. The most widely used solvent is isopropyl alcohol (isopropanol/IPA). For general localisation of lipids, Oil Red O is followed by a nuclear stain. Also, the tissue once stained, can be shown by polarised light to demonstrate birefringement cholesterol.