Figure 1. Schematic view of peroxisome biogenesis in mammals. The intracellular locations and molecular properties of peroxins are shown. Peroxins are divided into three groups: (i) peroxins that are required for matrix protein import; (ii) those, including Pex3p, Pex16p and Pex19p, essential for peroxisome membrane assembly; (iii) those such as Pex1p and Pex6p of the AAA family that may be involved in membrane fusion step and (iv) those such as three forms of Pex11p, Px11p, Pex11p and Pex11p, apparently involved in proliferation. Import of matrix proteins has been better understood: matrix proteins PTS1 and PTS2 are recognized by Pex5p and Pex7p respectively. Two isoforms of Pex5p are identified in mammals. PTS1 proteins are transported by homo- and heterodimers of Pex5pS and Pex5pL to peroxisomes, where Pex14p functions as a convergent, initial docking site of ‘protein import machinery’. Pex5pL directly interacts with the PTS2 receptor, Pex7p, carrying its cargo PTS2-protein in the cytosol, and translocates the Pex7p–PTS2 protein complex to Pex14p. Pex5p, carrying the cargos, subsequently translocates to other components such as Pex13p, Pex2p, Pex10p and Pex12p. PTS1 and PTS2 proteins are then released at the inner surface and/or inside of peroxisomes. Both Pex5p and Pex7p finally shuttle back to the cytosol.