shown. They may follow a co-translational insertion route that utilizes stromal ribosomes and the SecY/E transmembrane complex, the SRP-dependent pathway, the spontaneous insertion pathway, or the GET pathway. The last of the three are post-translational pathways originating from nuclear genes and therefor constitute the majority of proteins targeted to the thylakoid membrane. According to recent review articles in the journal of biochemistry and molecular biology, the exact mechanisms are not yet fully understood. Both chloroplasts and mitochondria Many proteins are needed in both mitochondria and chloroplasts. In general the dual-targeting peptide is of intermediate character to the two specific ones. The targeting peptides of these proteins have a high content of basic and hydrophobic amino acids, a low content of negatively charged amino acids. They have a lower content of alanine and a higher content of leucine and phenylalanine. The dual targeted proteins have a more hydrophobic targeting peptide than both mitochondrial and chloroplastic ones. However, it is tedious to predict if a peptide is dual-targeted or not based on its physio-chemical characteristics. Peroxisomes Peroxisomes contain a single phospholipid bilayer that surrounds the peroxisomal matrix containing a wide variety of proteins and enzymes that participate in anabolism and catabolism. Since it contains no internal DNA like that of the mitochondria or chloroplast all peroxisomal proteins are encoded by nuclear genes. To date there are two types of known Peroxisome Targeting Signals (PTS): Peroxisome targeting signal 1 (PTS1): a C-terminal tripeptide with a consensus sequence (S/A/C)-(K/R/H)-(L/A). The most common PTS1 is serine-lysine-leucine (SKL). The initial research that led to the discovery of this consensus observed that when firefly luciferase was expressed in cultured insect cells it was targeted to the peroxisome. By testing a variety of mutations in the gene encoding the expressed luciferase, the consensus sequence was then determined. It has also been found that by adding this C-terminal sequence of SKL to a cytosolic protein that it becomes targeted for transport to the peroxisome. The majority of peroxisomal matrix proteins possess this PTS1 type signal. Peroxisome targeting signal 2 (PTS2): a nonapeptide located near the N-terminus with a consensus sequence (R/K)-(L/V/I)-XXXXX-(H/Q)-(L/A/F) (where X can be any amino acid). There are also proteins that possess neither of these signals. Their transport may be based on a so-called "piggy-back" mechanism: such proteins associate with PTS1-possessing matrix proteins and are translocated into the peroxisomal matrix together with them. In the case of