Level M vectors are similar to level 2 vectors, but have a BsaI site located upstream of the two inverted BpiI sites. When one or several genes are cloned in a level M vector, a second BsaI is added at the end of the construct via a Level M end-linker (ref). This allows a fragment containing all assembled genes to be excised from the vector and subcloned in a next level of cloning (Level P).