24th October 1966.

  ':,'?fi i-.-err very plcr3zcd to ,r;et your letter.
convincing indeed and w are. all eagerly     CO~:;;;:oi~e~t 3G looks very

found tl:ie 3 protein t."Lps.          tvaitic:; to hear that you have

  :':c hsve looked u? the z:ap:pin,T data on FC 58 and v:e at;ree wit:1 you
that Were is something fis.9y about it.
special derivative of 1589.          Leslie crossed FC 58 with a

the                 This is x25 + 1539.
  A cistron close to the beginning of 1589.      X25 is an ochre in

cistron and thus                       This knocks out the A
         should rake the mapping easier.   The cross with FG 58
shoxed (a smal3. nuuber of rccombinants although this was not done quantit-
atively.
positive  Yhat is ambarrassinz ho:yever is that J?C 201 gives a clear
     result on spot CTOSD with x25 + 1589.
under 1231.                           In addition FC 201 lies
      This and all the other data for the nutants in that region
suggest that FC 201 is to the left of FC 58
give a strong positive                in 17M.ch case FC 58 should
          result :irith X25 + 15fi9.
unable to explafn this anomaly.         At the moment we are

  If thrngs had turned out as we expected we
send you a copy of the completed paper.      should have been `able to
                                  It has all been completely
typed and all the figures and references etc. finished.   The only remainin;
thing that had to be done vJas to check an entry in one 01 the tables.
was to make sure that X655 did not revert to ochre.                  TM Ir
                                    Vhen Leslie checked

this IY~ were astonished and delighted to find that it did.   This made us
suspect that our previous attempts to revert some of thembarriers in this
way might have &en inadequate.
.b6 also revert to ochres,      She has just found that barriers b5 and
class of nonsens0 triplet,  This makes it extremely likely that our third
is UGA.          which previously we had been unable to identify,
    We are now exploring this intensively and have decided to hold up


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24th October 1966,



the paper until It is all sorted out 89 we shall have to rewrite one
or two sections which deal with the nature of the barriers this way,
However,  I have asked Leslie to send you a copy of the map and we will
let you h*>vc the complete paper in its revised ford in a few t?lteeks time..
          6`
    I hqve also asked her to send you the folloning mutantst-             '.
                                                                           \
                X25 + 1589
                  X655
                 PC (l+ 222)

FC 222 is the filus mutant at the extreme right of our region,   It, give3 .,
a minute when combined with FC 1,                                

   If there is any other inEormation you w?ct to kmw in relation to
mutmts th?rit you would like us.to send you pl~r;o do not hesitate to
ask.   As you can imagine Leslie is very occupied just at the moment so
I doubt if she Wll look into the mapping anorzaly of iT 58.  Do let ug
know anything that you find about it:

F,H.C, Crick.




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