Cargo leaves the ER for the Golgi in vesicles/tubules bearing the coat protein COPII. The COPII coat contains polypeptides such as Sec23-Sec24 dimers and Sec13-Sec31 tetramers, and these play various roles in cargo sorting and vesicle formation. Sec31 was first identified in yeast, and a mammalian orthologue (SEC31A) has been characterised. Jon Morrow and co-workers now reveal a novel form of mammalian SEC31 - SEC31B - which has numerous variants that could create different COPII coats for different cargos (see p. 958). The authors find that SEC31B is widely expressed but undergoes massive alternative splicing. The splice variants fall into two main categories: full-length forms (SEC31B-F) and truncated forms (SEC31B-T) that lack the C-terminal proline-rich region. Morrow and co-workers demonstrate that SEC31B-F binds to Sec13 and colocalises with SEC31A and other COPII components in the ER and vesicular tubular clusters (VTCs). SEC31B-T associates with the ER but has a slightly different distribution. Since SEC31B-F coimmunoprecipitates with SEC31A and its partner SEC13, the authors propose that the SEC13-SEC31 tetramer can be built with different SEC31 isoforms that modify the coat to accommodate different types of cargo, such as collagen and GPI-linked proteins.