I performed in vitro DNA replication. and run the product on 15 % urea polyacrylamide gel after 5min denaturation in 95 oC. I have been doing it for several times and works fine, however, recently I start seeing a ghost band or smear that doesn't refer to any size product ( the upper smear in the attached picture). kindly note the my primer is Cy5 labeled. I am wondering what might cause this band. I was thinking that there might be some proteins interacting with DNA but I already treat the product with Proteinase K for 30 min. the left lane is a control, DNA only without any protein, I should see only one band but still the ghost band appears.

Can you please suggest me how to eliminate it and where does it comes from ?

Thank you