In most cells the smooth ER area is scare. Instead there are areas where the ER is partly smooth and partly rough called the transitional ER. The transitional ER gets its name because it contains ER exit sites. These are areas where the transport vesicles that contain lipids and proteins made in the ER, detach from the ER and start moving to the Golgi apparatus.[1]
  1. ^ Alberts, Bruce; Johnson, Alexander; Lewis, Julian; Raff, Martin; Roberts, Keith; Walter, Peter (2002). Molecular biology of the cell (4th ed. ed.). New York: Garland Science. ISBN 0-8153-3218-1. {{cite book}}: |edition= has extra text (help)