Nov. 10, 1947.
Dr. Robert Guthrie,
National Institute of Health
Bethesda 14, Maryland.
Dear Guthrie,

I have your postcard asking about reversion in E. coll. We
haven't published anything about this yet. F.J.Ryan, Lept.doology,
Columbia University, is doing most of this work, and he has reviewed
some of it in his paper in the 1946 Cold Spring Harbor Symposiun.
About all there is to say is that reverse mutation does take
place, that the reversion of different mutant loci in a multiple
mutant is mutaally independent, with rates of the order 10°76 to
10-8 per cell per generation in most cases. The number of reverse
mutants in a series of similar cultures shows the same high anormal
variance that Luria arn Delbrtick describe. Finally, one reversion,
from lactose-negative to lactose-positive has been analysed genetical}
and proven to be a reverse mutation. Reversion is evidently a spon-
taneous process; obviously the composition of 4 population will be
greatly influenced by the relative survival values of the mutant and
its reversion.

Howard Mason tells me you have quite a fey mitants in strain 58

by now. I would appreciate 1t if you could let mw kmow whether you
would be willing to release any or all of these for my collection.

Best regards, _ Sincerely,

Joshua Lederberg
Ass't Professor of Genetics.