In resistant F4-6 cells increased expression of mdr-genes (Fig. 2 right), and in wild-type adriamycin-sensitive F4-6 cells mdr transcripts at scarcely detectable levels could be clearly demonstrated. This position corresponds to the size of the mdr1b mRNA (Hsu et al., 1989). In comparison, in the adriamycin-resistant cells, transcripts ranging from 4.5 to 6 kb reacted very strongly with the mdr1 cDNA probe. As discussed previously (Schaefer et al., 1993), the combined message obviously results from the crossreaction of the human mdr1 cDNA probe with the mouse mdr1a and mdr1b mRNAs and from the heterogeneity in the size of mdr1a transcripts (Hsu et al., 1989; 1990).

It is apparent that BVDU + adriamycin treatment led to mdr transcripts at more than five times lower levels than treatment with adriamycin alone, and that BVDU  inhibited the formation of  mdr-gene amplification. In reality the preventing effect of BVDU was much higher than is expressed in the lower levels of mdr transcripts because the cells that did not undergo mdr-gene amplification were killed during treatment and could not be detected. At the end of treatment only cells survived that were resistant at least to a certain extent. Therefore, the more relevant  effect is the inactivation of cells seen in Fig. 1 A.