Using PCR technique, the two nucleotides are used as primers to amplify SSX2 DNA sequence using pET9a24aSSX2 as a template. By using these primers, the restriction sites and six His-tag genes are appended onto SSX2 genes giving rise to Nde1-NHSSX2-Not1 DNA fragments. After PCR, Nde1-NHSSX2-Not1 is presented in the form of double-stranded DNA fragments. Then, Nde1-NHSSX2-Not1 fragments are cloned into a cloning vector, pCR2.1 plasmid, to create plasmid pCR2.1-Nde1-NHSSX2-Not1. What is the purposed of this step? Explain why the researchers insert the gene in the cloning vector (pCR2.1) not in the expression vector (pET9a24a).