Spontaneous origin of resistance to DAC. (A) Two subclones of the HL60 cell line. HL60D developed a heterozygous 454C>G point mutation of DCK. This mutation was absent in another batch of HL60 obtained from ATCC. (B) The HL60D cell line had a growth advantage over the HL60 cell line. Cell number was counted from day 1 to 7. (C) Heterozygous deletion in exon 1 of DCK in HL60R cells. (D) Deletion was preexisting in HL60D cells and absent in HL60 cells. We designed a set of primers that spanned the deleted region and amplified that region from parental HL60D cells. (E) HL60R cells transfected with DCK restored sensitivity to DAC. We transfected wild-type DCK into HL60R and selected stably transfected cells by G418. HL60R and DCK-transfected (HL60T) cells were treated with DAC at 0.02, 0.2, 2, and 20 μM, respectively. Cell viability was counted. (F) Transfection of DCK cDNA restored dCK protein expression. dCK protein expression was measured in HL60R and HL60T cells by Western blot analysis.