"Example of primer designed to add a 6xHis-tag using PCR". Eighteen bases coding six histidines are inserted right after the START codon or right before the STOP codon. At least 16 bases specific to the gene of interest are needed next to the His-tag. With 6 His, the protein will have an added 1 kDa of molecular weight. Often, a linker (such as gly-gly-gly or gly-ser-gly) is placed between the protein of interest and the 6 His tag in order to prevent the polyhistidine tag from affecting the activity of the protein being tagged.