TKT and neoplasm: Interestingly, after 42 to 46 days of oFV-TK amplification in subcutaneous U251-U3-mCherry-U3-luc tumors in vivo majority of the proviruses detected in the chromosomal DNA isolated from the infected tumor cells still carried intact TK cDNA (Figs. 4B, C), indicating minor transgene loss.