Rijswijk, 22 August 1975
Dear Harold,
The BALB/c line I sent you will get about 40 % mammary tumors when bred after 15 months average.  In about 10 % of the tumors I found infectious MTV.  In a considerably higher percentage of tumors we do find MTV-P28 with our new sensitive immunoassays.  When we pass this virus in BALB/c it induces 100 % tumors at 6 months.  The virus then behaves as a traditional milkfactor.  If you want to have some mice of the line with passaged MTV-O I can send you them. [HANDWRITTEN NOTE: sent in milk for comp[?] assay]  We also have a lot of tumors induced by this virus.
As you know we claim that we can induce MTV in normal kidney cell cultures by exposure to dexamethasone or DMSO.  Most of our work was based on immunofluorescence and among other Hilgers cannot confirm this.  We have, however, also EM data.
We have now found that supernatants of C3Hf kidney cultures treated with either drug induce tumors at 5 months in BALB/c mice.  As you know C3Hf mice in vivo only release MTV-L, and therefore we are excited about this finding.  Of course, I had an open mind for cell or virus contamination in the tissue culture lab, but I have found it now in three consecutive experiments.  I regard contamination as a very remote possibility.
The review is almost finished.  Thank you for your permission to quote your on the higher number of MTV-DNA copies in the GR.
With kind regards,
P. Bentvelzen