Most skin biopsies don't have global periods, so no modifier should be required for the treatment.
If the biopsy is one with a global period, and the treatment date is within 10 days of
the date the sample was taken (not from when the results came back), then the modifier used will depend on the CPT code used to describe the treatment.
You'd need a 24, if it's an E/M, or 58 if it's a procedure code. Hope that helps!