Tissues can be a decent source of DNA evidence, the New York Times' Claiborne Ray writes in response to a reader question.
Manuals about collecting DNA evidence say that facial tissues "can be a very good place to look for cells with human DNA, whether they are in mucus, blood, sweat, semen, or ear wax."
Ray notes that the manuals also say that any wet evidence has to be air-dried, and that moisture can lead to sample degradation as it encourages bacterial and mold growth. Newer amplification techniques, she adds, can still lead yield evidence from samples that have been contaminated by mold.
After drying, she notes that the DNA sample should be stored in a non-permeable container out of direct sunlight and in a spot with low humidity.
We do wonder, though, what the questioner has been getting up to with his or her tissues.