June 8, 1956

Dear Mrs. Elliott:
Thank you for your note of the 4th.

The matter of 1633A is certainly of more sentimental (to m)
than schantific interest. I worked on the "adaptations" of this
strain, among others more than ten years ago, and I am afraid it would
be quite a chore to dig all the details out of my notes. (Some of
that work was published with Ryan, in PNAS 32h163). I think the following
is a fair stapment:

Iwas specifically looking for reverse-mtation of pab and set up
quite a number of long-term selections. 1633A proved to "adapt" on minimal
medium in quite a few tests, but the ability to grow on minimal was rarely
transmitted through the conidia. I did have one case of an apparent reversion
and another where backcrosses of a single spore pab+ isclate gave pab-
progeny. This might have been dueto mtational instability, incompetence,or
a suppressor and I would not want to insist on which one. The point does
not seem a vital one, and I would rather you did not make any definite
attributions. Why don't you merely omit the "so far as is known" story al-
together, and just say that you never saw anything that would lead you to
suspect the marker? I am rather surprised myself that you never ran into
the so-called "adaptations", whatever they are.

Yours sincerely,

Joshua Lederberg
Professor of Genetics