Four cells that lack the protein PERK observed under a fluorescence microscope. The endoplasmic reticulum (in red) is the protein folding factory of the cell, located around the nucleus (in blue) that contains the DNA. The green strings are filaments of actin that make up the cell skeleton. In healthy cells, PERK would modify the cell skeleton to move the endoplasmic reticulum towards the plasma membrane. In the cells shown here, however, the cell skeleton has an abnormal shape, so that the endoplasmic reticulum cannot make proper contact with the plasma membrane.