21st February 1967.
Dear Severo,
Thank you for your letter of 13th February.  We were very interested to learn that UGA gives negative results with both tryptophan and cysteine.  Our present idea for the end and beginning of cistrons is UAAUGA, it being assumed that the new cistron starts with the AUG read out of phase.  We would only expect this to occur between cistrons which were read with equal efficiency.
We were also interested to hear that you can produce formylmet-phe in good yields in the cell-free system.  Mark Bretscher had tried AUGUAA in the hope that it wound release formylmethionine, but it only did so extremely poorly.   More recently he has been working with the amber fragment at the beginning of S2.  However we have so far been quite unsuccessful in finding the hypothetical chain terminating tRNA, and wil1 be interested to learn whether you have had any better luck than we have.
We should all be delighted to see you here in April.  Either the 27th or the 28th would be equally convenient.  Perhaps you would let me know nearer the date which day you prefer and whether you would like us to make a hotel reservation for you.  I do not know whether you would care to give us a formal talk, but we would be quite happy for you to chat to us informally if you would prefer that.
Apart from the work on UGA the only important result we have here is that the alteration to the tyrosine su+ tRNA is the expected one in the anticodon.  We are hoping to pick up other mutants in this particular tRNA but the work has only just got under way so we have no more results yet.
Greatly looking forward to your visit,
F.H.C. Crick.