This stage is after the adsorption of the adapter-ligated fragments on the flow cells. The fragments are isothermally amplified to generate clusters [1]. The exact nature of the enzyme mix that catalyses this step is not disclosed by Illumina. **I assume** it must be similar to **Helicase mediated isothermal amplification** , which uses helicase and single strand-binding proteins (SSB) to denature the template. The polymerization is catalysed by a mesophilic DNA-Pol such as Klenow ( _E.coli_ ).