diff --git a/collects/typed-scheme/private/base-env.rkt b/collects/typed-scheme/private/base-env.rkt
index 11f7a99265..d2e869e84a 100644
--- a/collects/typed-scheme/private/base-env.rkt
+++ b/collects/typed-scheme/private/base-env.rkt
@@ -13,6 +13,7 @@
  (for-syntax (only-in racket/private/pre-base new-apply-proc)
              #;racket/string)
  scheme/promise scheme/system
+ racket/mpair
  (only-in string-constants/private/only-once maybe-print-message)
  (only-in mzscheme make-namespace)
  (only-in racket/match/runtime match:error matchable? match-equality-test)
@@ -912,4 +913,9 @@
 ;; probably the most useful cases
 [curry (-poly (a b c) 
 	      (cl->* ((a b . -> . c) a . -> . (b . -> . c))
-		     ((a b . -> . c) . -> . (a . -> . (b . -> . c)))))]
\ No newline at end of file
+		     ((a b . -> . c) . -> . (a . -> . (b . -> . c)))))]
+;; mutable pairs
+[mcons (-poly (a b) (-> a b (-mpair a b)))]
+[mcar (-poly (a b) (-> (-mpair a b) a))]
+[mcdr (-poly (a b) (-> (-mpair a b) b))]
+[mpair? (make-pred-ty (make-MPairTop))]