(A) Wild-type RARα and PML-RARα as well as mutant PML-RARα and RARα were transiently transfected into Cos-1 cells. Nuclear extracts of transfected cells were subjected to Western analysis to examine the expression of exogenous proteins. Lane 1, Cos-1 cells transfected with mock; lane 2, transfected with RARα wt; lane 3, transfected with mutant RARα (RARα m4); lane 4, transfected with PML-RARα wt; lane 5, transfected with mutant PML-RARα (PML-RARα m4). (B) Specific nuclear tRA binding activity in Cos-1 cells transfected with (1) PML-RARα m4 or (2) RARα m4 in comparison to those transfected with wild-type receptors (3) and (4). Nuclear extracts were incubated with [³H]-tRA alone (•) or with [³H]-tRA in the presence of 200-fold excess of unlabeled tRA (○).