Illustration showing the proteins and other molecules constituting the cell membrane. Proteins can associate with the membrane in a few ways. Integral membrane proteins are a permanent part of a cell membrane and can either penetrate the membrane (transmembrane) or associate with one or the other side of a membrane (integral monotopic). Peripheral membrane proteins are transiently associated with the cell membrane through anchoring molecules. Membrane proteins constitute a significant proportion of human proteins, making them a key target for pharmaceutical drugs.