There are two ways to add polyhistidines. The most simple is to insert the DNA encoding the protein in a vector encoding a His-tag so that it will be automatically attached to one of its ends [//upload.wikimedia.org/wikipedia/commons/0/0c/His-tag.png (See picture)]. Another technique is to perform a PCR with primers that have repetitive histidine codons (CAT or CAC) right next to the START or STOP codon in addition to several (16 or more) bases from one end of the DNA encoding the protein to be tagged (see primer example below).