I am needing to identify BL6 mouse pups P4 or younger for 2-3 days. Permanent sharpie markers seem to last 1 day, if that. Does anyone have any better suggestions?
You can use tattooing. If you don't have access to a tattooing machine, I have seen commercial vendors use toe clipping. It is explained in this article.
Article Developmental and Behavioral Effects of Toe Clipping on Neon...