June 26, 1969
Dear Doctor Singer:
Thanks for your letter of June 18th.  If I had known of Heppel's paper at the time I wrote our note, you can be sure I should have quoted it together with the other papers listed in my publication.  As you certainly know, it is almost impossible to keep track of the literature at the present time.
I have not yet had time to read this paper, but I am wondering whether the situation which we describe is not different from the one prevailing when you deal with a ribopolymer and a mononucleotide precursor in the polymerization reaction.  In the latter case, even if polynucleotide phosphorylase is not a template-requiring enzyme, the ribopolymer may act as a sort of template.  For such reactions, there are, of course, many precedents in the polymerase systems.
As soon as I have reprints, I shall send you one.
With kindest regards,
Sincerely,
Erwin Chargaff