August 7, 1956

Dr. E. Kellenber ger

Laboratoire de Biophysisue

Universite de Geneva
Switzerland

Dear Dr. Kellenber ger:
Thakk you for pour letter of duly 14.

The Lp types mentionedm in the paper have been and are being studied
further. The immune phenotype is associated with the he terogenotic state,
and accaérding to our present working hypcthesis corresponds to a genotype
in which the Lp® factor 4s carried both in the fragment and in the chromosome.
Alternatively, lysogenic heterogenotes are believed to carry Lp* in both
sites. A number of additional experiments are needed, and some are under
way, to test this hypothesis. The main evidence for it is that all haploid
(non-heter ogenotic) segregants from the Immune clonesaare sensitive; conversely,
of the heterogenotic derivatives from the same clones remain immune.

The occurrence of these imaune types is fairly typical of transductions
to sensitive recipients, when these can be carried out at low miltiplicity
(i.e., with "HFT" lysates). Dr. Morse has been undle to find any evidence
of produétion of lambda from these immnes, in distinction to ous and others!
experience with other"defective prophages". If you would be interested in an
electron-microscépic study of this situation, we will be obliged to hear of
your results. Under separate cover, Mrs. Lederberg is sending ~518 which
will serve as well as any other culture for the demonstrdion, as remested
in your letter.

We thank you for sending us reprints of your studies, and hope the ex-
change will be continued.

Yours sincerely,

Joshua Lederberg
Professor of Genetics