The 4X Laemmli sample buffer is a critical component in the preparation of protein samples for SDS-PAGE, ensuring the resolution of protein bands. Its composition typically includes Tris-HCl, glycerol, SDS, and a tracking dye like Bromophenol Blue. In the reducing Laemmli sample buffer, a reducing agent such as β-mercaptoethanol or dithiothreitol (DTT) is added to break disulfide bonds within protein subunits, allowing them separate by size in electrophoresis.