duly 21, 1949.

Mr. Norton D. Zinder,

Dept. Genetics,

Carnegie Institution,

Cold Spring Harbor, Lele, N.Y.

Dear Norton:

Your letter received, and the cultures you wanted sené out immediately.
I hope that they are all atill viable. If not, send me a card ami I'll re-
place thea.

Although EMS—Lac ie probably the medium of choice for experiments in
linkage determination, asparagine is not really essential for T(0), and for
that matter, you probably would do quite as well with "M-9" in your crosses.
Don't forget the "Linkage of to BM" if you want large yields of recos-
binants to do alleliam tests with.

fo help interpret the heterosygotes, I am badly in need of markers located
within 10 or 15 unite of Lac. If you should stumble onto any such, I would
appreciate it if you could ask Dr. Demerec to have them gent to me. Except that
some regular mechanism of elimination of segments igeluding the Mal, and Gel,
looi is operating, there is no further olarification.

Don just came by to mention that SH-13 was not included in the package just
sent. It will be forwarded promptly.

Your let clase mil, if any, is being forwarded. You have a bit of 2d class
matter: "Genetics", and propaganda from Columni. Do you want these forwarded?

In reference to lysogenicity, Dr. Stan Shapiro is working in the lab for a
fen weeks, and has been crosa-testing with Don some 40 E. coli isclates from
foal. intestines. About 30% of the cultures are lysogenic for one or more of
the other cultures in the group. Some of these lysogenic phages appear to be
quite "strong", and should better material than lambda or the Salmonella
phages for this study.

I hope that you can have your report without too auch delay, and that you
will find time to reasin in touch with us here.

With best regards,
Sincerely,

Joshua Lederberg