UNIVERSITY of PENNSYLVANIA

PHILADELPHIA 4
October 18, 1956

The School of Medicine

DEPARTMENT OF PEDIATRICS
1740 Bainbridge Street
Philadelphia 46, Pa.

Dr. Joshua Lederberg

Department of Genetics

University of Wisconsin College of Agriculture
Madison, Wisconsin

Dear Josh:

Thymine is incorporated into 15—= but very poorly into B. I
have no information on K 12. However strain 15 is relatively inert
to thymine analogues suggesting it does not incorporate but of course
15y- does, Therefore one can't prejudge the mtant progeny too
completely from the properties of the parent, at least with respect
to this function. The sulfa trick permits the exposure of a thymine
route in B which incorporation data did not,

Gunther Stent did some tricks with the sulfanilamide tekck
on Kl2 but I can't remember what he obtained. You might write
to him about it.

Thanks for the reprints.

With best regards to Esther and yourself,

Sincerely yours,
Rp

i

Seymour \Cohen

SSC/hr