Research using more challenging 12 and 24 fragment assembly test systems showed the clear superiority of the recently re-engineered BsaI-HFv2 enzyme over BsaI. This is evident in efficiencies of assembly (number of transformants), accuracies of assembly (fidelity) and continued increases in assembly formation at higher than usual cycle numbers if desired. The mix is identical to the original except for the Type IIS restriction enzyme replacement.