PML/RAR expression in NB4.007/6 cells restores RA sensitivity. (A) NB4 and NB4.007/6 cells were treated with RA for 12 hours and then further cultured in RA-containing medium, either in the presence or in the absence of 50 μmol/L LLnL, as indicated. At the end of treatment, TGase II activity was measured in the cellular extracts. (B) NB4.007/6 cells were infected with retroviruses expressing GFP alone (GFP), GFP-RAR (GFP-RAR), or GFP-PML/RAR (GFP-P/R) and then treated for 5 days with RA. At the end of the treatment, GFP-positive cells were analyzed for expression of the differentiation antigens cd11b and cd11c. The white bar (NB4) refers to the uninfected, control NB4 cells after 5 days of RA treatment. These results are derived from a single experiment of three that gave comparable results.