PEX19 is a chaperone and import receptor for newly synthesized, class I peroxisomal membrane proteins (PMPs). PEX19 binds these PMPs in the cytoplasm and delivers them to the peroxisome for subsequent insertion into the peroxisome membrane, indicating that there may be a PEX19 docking factor in the peroxisome membrane. The human peroxins PEX3 and PEX19 are essential for peroxisome biogenesis. They mediate the import of membrane proteins as well as the de novo formation of peroxisomes. The PEX3 membrane protein acts as a docking site for PEX19, a cytosolic chaperone for PMPs that delivers PMPs to the endoplasmic reticulum or peroxisomal membrane. PEX19 binds newly synthesized peroxisomal membrane proteins post-translationally and directs them to peroxisomes by engaging PEX3, a protein anchored in the peroxisomal membrane. After protein insertion into the lipid bilayer, PEX19 is released back to the cytosol.