FAQ: Can I use a 15-nt overlap that is entirely composed of His-tag repeats (i.e., CACCACCACCACCAC)?

No, you must flank the His-tag sequence on both sides with at least 2 nucleotides that are not part of the His-tag repeating sequence. Alternatively, intersperse CAC and CAT his codons to interrupt this repetitive sequence. You should avoid repeating sequences at the end of an overlap.