NEBNext Q5 Hot Start HiFi PCR Master Mix is the original hot start NEBNext formulation of Q5 High-Fidelity DNA polymerase. This formulation is also included as a component in all of the original NEBNext Ultra kits, for DNA and RNA library preparation. However, the NEBNext Ultra II Q5 Master Mix is now available, and this more recent hot start formulation further improves the uniformity of amplification of libraries, including superior performance with GC-rich regions.