All nuclear-encoded mRNAs are translated on cytosolic ribosomes. Ribosomes synthesizing nascent proteins in the secretory pathway 1 are directed to the rough endoplasmic reticulum (ER) by an ER signal sequence 2 . After translation is completed in the ER, these proteins move via transport vesicles to the Golgi complex dlccirc3; from whence they are further sorted to several destinations 4a, 4b, 4c . After synthesis of proteins lacking an ER signal sequence is completed on free ribosomes 1 , the proteins are released into the cytosol 2 . Those with an organelle- specific uptake-targeting sequence are imported into the mitochondrion 3a , chloroplast 3b , peroxisome 3c , or nucleus 3d . Mitochondrial and chloroplast proteins typically pass through the outer and inner membranes to enter the matrix or stromal space, respectively. Some remain there, and some 4a are sorted to other organellar compartments. Unlike mitochondrial and chloroplast proteins, which are imported in a partially unfolded form, most peroxisomal proteins cross the peroxisome membrane as fully folded proteins 4b . Folded nuclear proteins, often in the form of ribonucleoprotein particles, enter through visible nuclear pores by processes discussed in Chapter 11 4c.