April 28, 1950.

Dy. Stan Shapiro,
Dept. Bacterioilory,
Iowa State College,
“mea, Iowa.
Year Stan: |

Your posteard shows an incredible dezree of optimlem. Were your 36 (1)
#ifferent combinotions ef mutants all fron the same strain? If they are,
my (unsolfceited) advice would be to concentrate on different straine for
further tests. Praotieally any sosbination of mitents from B. coli K-12
will show recombination easily, whereas ether strains will wot. Since the
specifictty seems to be primarily between strainga rather than between mutants
of the same otrain, we have been follewing the policy with Salmonella of inves-
tdcatine only a fev mutants fron ariven etrain, in omer to cover as many
different strains as possible. There are a number of well-defined serobacter
strains (e+c. clocene) which are used commercially, and are centrally available.
In order to avoid sorocone else's future duplication, I have felt thet it
ie best to survey such wolledefined typea firnt.

Unfortuhctoly, wo haven't planned to go to Baltimore, so we won't be
able tosses you thoro. But both Gowen and Yerknan are visiting here next
week, so wo'll be seeing quite a bit of Tova.

Have you roceived "!‘dorobial Genetics Bulletin" yet? I've asked Dr.
Witkin at Cold Spring Harbor te put you on the mailing liat.

Esther servis her best, Sincerely.
Joshua Lederbers