ER MCSs with other organelles and the PM. ER MCSs with other organelles and the PM. The ER forms MCSs with mitochondria, Golgi, endosomes, peroxisomes, lipid droplets, and the PM. These MCSs are closely opposed but not fused membranes containing various molecular machineries. Factors localized to these MCSs mediate essential cellular processes, including lipid and ion exchange, organelle positioning, and biogenesis. Haoxi Wu et al. Science 2018;361:eaan5835 Copyright © 2018, American Association for the Advancement of Science