Overexpressed SMRTβ isoform promotes ligand-induced transactivation of PML/RARα mutant I410T in ATRA-resistant NB4-MRA1 APL cells and other cell lines. Transient cotransfection assays to determine the effects of SMRTβ and SMRTα overexpression on I410T activity using a DR5-tk-CAT reporter gene in (A) ATRA-resistant APL NB4-MRA1, (B) hematopoietic Jurkat, (C) hematopoietic U937, and (D) nonhematopoietic Cos-1 cell lines. Cells were cotransfected either with empty expression vectors pCMX or pSG5 (control) or with expression vectors for PML/RARα I410T and specific vectors for SMRTβ or SMRTα. Assays were performed in triplicate and repeated at least 3 times. Bars represent standard deviation of the mean. Numbers in parentheses indicate the fold induction produced by ATRA. There was a significant difference (^**P < .01) for fold induction of transcription in cells transfected with SMRTβ compared with control transfection (A-C), and there is no significant difference between cells transfected with SMRTα. In Cos-1 cells (D), SMRTα-transfected cells showed a significant decrease (^*P < .01) in induction relative to control- or SMRTβ-transfected I410T.