Illustration of the synthesis of proteins that are bound for secretion from the cell. Ribosomes (white) produce proteins (chains of spheres) from an mRNA (messenger ribonucleic acid, white and blue strands) template. This process is known as translation. When proteins are to be secreted from a cell they begin with a signal peptide (4) that is recognised by a signal recognition particle (SRP), which binds to a translocon (orange) in the membrane of rough endoplasmic reticulum (RER). Once attached, protein synthesis resumes, with the signal peptide detaching from the growing protein, which passes into the lumen of the RER. When the protein is complete the ribosome detaches from the RER and disassembles.