Columbia Gniversitp
inthe Citp of Hew Pork

DEPARTMENT OF ZOOLOGY

21 February 1946 .
Dear Dr. Tatum:
Sorry I missed you yesterday.
Progress Report:

A second mthod for mutant detection has been worked
out, and it seems, and is surefire (except that no de novo
mutants have been detected with it yet. It consists simply in
plating the mixture of wild type (prototroph) and mutant
bacteria into a minimal agar. The prototrphs grow up and
form good sized colonies in about 24 hrs. The mutnts, by
definition.d6 not grow. Then pour a layer of some complete
medium (like Yeast Dextrose Agar) on the plate. (I use
about 5-7 ml.) In from 8-12 hours the muita nt colonies
will appear, and can be distinguished ffom the prototrophs
simply by their size at this time, br by having mked the
others at their first appearance. By 24 hrs, the mutant
colonies will be good sized and should be pickable. The
possible applications of this technique are obvious. It
should be possible to add the supplement in liquid form,
which may be more convenient.

Sincerely yours,

Joshua Lederberg.

See P-5

WP che :