Poly(ADP‐ribose) polymerase (PARP) expression and inhibition, phosphatase and tensin homolog (PTEN) deficiency and drug‐transporter expression in PARP inhibitor (PARPi)‐resistant variants. A, protein levels of PARP1, PARP2, PARP3 and Tankyrase1/2 detected by western blotting in PARPi‐resistant variants and parental U251 cells. B, PAR formation induced by the DNA damage agent MMS (0.01%). C, PARPi inhibited the formation of PAR triggered by MMS (0.01%). D, DNA sequencing revealed the same homozygous c.720_721 insTT of PTEN existing in all 3 PARPi‐resistant variants and parental U251 cells. E,F, Protein levels of PTEN (E) and its downstream signaling factors (F) determined by western blotting. KB/VCR cells served as the positive control of PTEN expression (E). g, protein levels of drug transporters P‐gp and BCRP were detected by western blotting. KB/VCR cells served as the positive control. OP, olaparib; SP, simmiparib; TP, talazoparib