They are generated by a leader-primed mechanism of discontinuous transcription, whereby the polymerase first transcribes the noncoding leader sequence from the 3'end of the minus strand, the capped leader dissociates from the template and reassociates with a complementary sequence at the start of any one of the genes to continue copying the template through to is 5'end. Only the unique sequence at the 5 end is translated to give different proteins. Synthesis is also unique. The envelope protein, M, is directed exclusively to certain internal membranes, namely cisternae of the endoplasmic reticulum and the Golgi complex, as a result of which virions bud only from that region and not from the plasma membrane. Virions then transported in vesicles to the plasma membrane for exit from the cell by exocytosis.
Recombination occurs at a high frequency between the genomes of different but related coronaviruses.