CARNEGIE INSTITUTION OF WASHINGTON

DEPARTMENT OF GENETICS

COLD SPRING HARBOR, LONG ISLAND, N, Y.

August 2, 1946

Dear Lederherg,

The situation on K-12 is not very hopeful, since

the patterns of resistance of the mutants are not very
clear as yet. As you can imagine, in of@der to establish
which combinations of resistance occur, and which do not,
one must not only pick up a few mutants, but also test
very large number of independent ones and test them with
all phages. If one finds that a cémplex resistant phe-
notype occurs, one must not only make sure that the
simple corresponding ones also occur, but that the for-
mer looks like the superposition of the latters. I doubt
that we can go very far on this in the remaining two
weeks of work here. What we can do, if you want to

send your double mutants, is to make them resistant

to one phage each, so that you may look for sensitives
in your prototrophes. Reversion is likely to be rare
enough not to complicate matters.

I would like to have your double mutants from B/r, both
to study mutability rates, and also because Mark Adams
would like to use them in phage growth experiments in
order to see how suppression of growth factors affects
phage growth. I shall appreciate your sending me a slant
of each.

Best regarss,

SE hirin. -