May 8, 1946.
My dear Sheng,

Yuxk The problem that you indicate is an interesting one, and I
should be happy to assist you in any way that I can. However, I do not
have at my disposal stocks of 5531-15300-A. However, I do have both
4540-a and 5531-15300—a, which I have been using for other heterocaryon
experiments. I should be glad to send you these, but I mst tell you
that heterocaryons which I have made between these strains have had
no detectable color on either minimal or complete medium, Therefore it
is evident that the dose of Alb-2* in this combinathon is indeed very
mum small, md too small to be useful for your purposes. Since my experi-
ments will entail analysis of this heterocaryon in terms of its compo-
nents I do not believe that anything would be gained by its study in
another laboratory. I would suggest other combinations,

In the course of another investigation, I had occasion to study
the variability of nuclear ratios in combinations of the two albinos:
4637 and 15300, and found considerable variation in the pigmentation of
cultures derived from hyphal tip isolations. The same should hold for
monogenic comparisons at the 15300 locus, Since there is no assurance
that heterocaryesis is the only means of symbiotic association between
strains, I would recommend thata more feasible approach to your problem
would consist of isolating hyphal tips from 15300*- 153007 combinations,
in an attempt to find cultures which by their pigmentation seem to have
varying dosages, and analysing these heterocaryons by outcrossing. A
further complication should be mentioned: there are indications that 15300
may mitate rather readily to a dilute allele which has a distinct pink

color. I have given Dr. Ryan a sample of a recent isolate of such a stock,
Low dosage 15300* combinations mst be distinguished from this mutation.
However while the mutant is pinkish, kmm= such low dose combinations tend to be
Yellowish rather than pink.

If in view of my remarks you would still prefer the mitants you suggest
I shall be happy to send them to you; on the other hand I would suggest that
other combinations might be preferable, or the scheme I mentioned.

We are doing some studies now on the analysia of heterocaryons whose out-
may bear on the techniques of such analysis ; we shall keep you informed,
through Dr, Ryan of such developpents,

With best regards,

Sincerely yours,

Joshua Lederberg.