Newly-synthesized proteins from the endoplasmic reticulum are sorted by the golgi apparatus (center) according to destination, then packaged into vesicles for delivery. This process ensures that newly-minted proteins arrive just where they're needed. In this illustration of protein sorting in a kidney cell, erythropoetin, a protein responsible for stimulating red blood cell production (orange), is separated from other proteins (green) and packaged into secretory vesicles for release into the bloodstream. It will travel to stem cells in the bone marrow, where it will trigger these cells to produce red blood cells.