* A misspelling or abbreviation: It could be a shortened or incorrect version of a longer term. Could you please provide more context or information about where you encountered this term?
* A niche scientific or technical term: If you're working in a specific field (e.g., biology, chemistry, engineering), it could be a technical term specific to that field.
To help me understand what you're asking, please provide more details about:
* Where did you encounter this term? (e.g., a scientific article, a textbook, a website)
* What was the context surrounding the term?
With more information, I can try to decipher what "PNA-FISH" might refer to.