UNIVERSITY OF CAMBRIDGE   DEPARTMENT OF PHYSICS

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![.R.C. Unit

D r .  M .  '.:' . Nirenberg,
Department of Health, Sduc;>.tion,
and Welfare,
Bethesda 14,
M . D . ,  U.S.A.

29th January, 1962.

De.:t.r Dr. Nirenberg,

   Thank you for,your revised paper.  We, too,  thought we
ha,d evidence for degeneracy, as in our hands both poly (U,C)
and Poly (U,A) incorporate leucine, and the result is not
due to an impurity in the isotope.  However, the argument
obviously depends upon poly U not incorporating leucine, so
we retested this.  To our surprise and annoyance we found
thst our poly U does stimulate the incorporation of some
leucine.   The amount varies but on one occasion it was as
hi&h as 25% of the phenyalalamine incorporated.

   Naturally we believe that the result with poly U is an
artefact, but we have not bpen able to track it down.   Until
we h::ve done this we don't feel we can trust the evidence
from the other polymers.

As far as we can see the result is not due to impurities
                      the polymers have

in our poly U.  Apart from the fact that
been xnalysed by Marianne we do not find
other amino acids which ::;e should expect
present in our poly U.

the incorporation of
if other ba.ses were

   I hope by the time we meet we shall
of the trouble.

have found the cause

Looking forward to discussing all this wit'n you.

Yours sincerely,

  f         .Y
  @p!f??
I?. H. C. Crick