system
Given the following post, respond as if you're commenting on Reddit:
human
Why do moles/birthmarks stick around if skin replaces itself every so often?

Also, how does increased exposure to the sun over a long period of time raise skin cancer rates if skin regenerates anyway?
gpt
Your skin contains several different types of cells.  The skin cells you're thinking of that regenerate and slough off every so often are called keratinocytes.  The cells that make up moles/birthmarks are a different cell type called melanocytes.  Melanocytes do not get replaced by keratinocytes.

As far as cancer goes, the skin regeneration is the problem!  UV light causes damage to cells by forming abnormal linkages in the DNA.  Often this damage can be corrected, but the more UV exposure, the more damage that occurs.  If it cannot be corrected, then it is possible that certain regions in the DNA are not read correctly during replication.  Some mutations can cause this new cell to become immortal, and it will keep on replicating.  This abnormal growth is cancer.