Common Lisp code to generate a list of strings

I wrote this bit of Lisp code. I'd appreciate comments for improvement, including comments about formatting, design (including choice of data stuctures) and performance. This was tested with sbcl 1.0.55.0 and written in emacs 23.2 with slime 20111027-2 on Debian squeeze. The formatting is therefore at least partly emacs formatting, though I did decide what should go on each line.

(defun inc (x endpos)
  ;;; Given a vector of the form V=(V_1, V_2, V_3,...V_k))
  ;;; with V_i in the set {"A", "C", "G", "T"}
  ;;; increment the vector. This behaves as though {"A", "C", "G", "T"} were the
  ;;; numbers 0, 1, 2, 3 in base 4. E.g. ("A", "C", "G", "T") increments to
  ;;; ("A", "C", "T", "A") is equivalent to ("0", "1", "2", "3") incrementing to
  ;;; ("0", "1", "3", "0")
  (let ((y (copy-seq x)))
    (case (aref y endpos)
      (#\A (setf (aref y endpos) #\C))
      (#\C (setf (aref y endpos) #\G))
      (#\G (setf (aref y endpos) #\T))
      (#\T (setf (aref y endpos) #\A)
       (when (> endpos 0)
         (setf y (inc y (- endpos 1))))))
    y))

(defun genstrings (stringlen)
  ;;; Generate all possible strings of length 'stringlen' containing
  ;;; the characters {"A", "C", "G", "T"}
  (let ((stringnum (expt 4 stringlen))
    (s (make-array stringlen :element-type 'character :adjustable t :initial-element #\A :fill-pointer t))
    (strlst (make-array 0 :element-type 'string :adjustable t :fill-pointer 0)))
    (dotimes (i stringnum)
      (vector-push-extend s strlst)
      (setf s (inc s (- stringlen 1))))
    strlst))