The　CRISPR/Cas12a　(cpf1)　system　was　reported　to　indiscriminately　cleave　single-stranded　DNA　after　binding　with　target　DNA　strands.　This　usually　required　the　target　DNA　strands　to　contain　the　protospacer-adjacent　motif　(PAM)　sequence　of　TTTN.　Herein,　we　found　Cas12a　can　also　recognize　another　PAM　sequence　of　UUUN　resulting　in　activation　of　its　ssDNA　collateral　cleavage　effect.　To　make　this　finding　useful,　by　combining　with　LAMP,　we　first　realized　CRISPR/Cas12a　for　directly　visualized　DNA　detection　at　the　single-copy　level.　By　treating　with　UDG　enzyme,　we　made　this　system　free　from　residual　amplicon　contamination,　which　is　a　big　problem　in　this　field.　Thus,　an　ultrasensitive　and　anticontaminant　DNA　detection　platform,　namely,　UDG　and　LAMP　and　CRISPR　(ULC).　This　new　finding　would　help　us　better　understand　the　mechanism　of　Cas12a　and　expand　its　application.　©　2019　American　Chemical　Society.