September 9, 1975
Dear Peter,
Thanks for your recent informative letter.  In case you're not going to Madrid, I thought I'd better write now to ask if you would still be willing, as your letter indicates, to send us some BALB/c mice into which you have introduced MMTV-O.  Even better, do you have any "high titre" milk you could send us?  We'd also like to look at a couple of tumors.
Gordon Ringold is trying to settle some questions about the homologies of the MMTV strains.  To date, it appears that all the sequences in MMTV-P are present in MMTV-S and MMTV-L, but it is possible that a small amount of the genome of MMTV-P (ca. 5-10%) is absent from MMTV-O (on the basis of competition hybridization using RNA from the spontaneous BALB/c tumors you sent us RNA from the S49 BALB/c lymphoma line which contains A particles).  (This assay, by the way, does not detect minor sequence differences; we see many such differences when MMTV-S and MMTV-P RNA are compared by T1 fingerprinting.)  To make the competition assay maximally sensitive we would like to prepare MMTV-O 70s RNA from the milk of your infected animals.  In addition, we would like to compare copy numbers in the DNA of spontaneous tumors and tumors from your infected mice, to relate these tumors to GR tumors (no increase over normal tissues).
Thanks for your generosity in these matters.  I'll keep you posted.
Sincerely yours,
Harold E. Varmus, M. D.
Associate Professor
Department of Microbiology