The peroxisome membrane contains import receptor proteins. Peroxisomal proteins, including new copies of the import receptor, are synthesized by cytosolic ribosomes and then imported into the organelle. Presumably, the lipids required to make new peroxisome membrane are also imported. (We discuss later how lipids made in the ER can be transported through the cytosol to other organelles.) In this model, peroxisomes are thought to form only from preexisting peroxisomes by a process of growth and fission.