Background The aim of this study was to identify the subnuclear distribution pattern of human orphan nuclear receptor steroidogenic factor 1 (SF-1) in living cells with and without the activation of protein kinase A (PKA) signal pathway, and thus try to explain the unknown mechanism by which PKA potentiates SF-1 transactivation. Methods Full-length cDNAs of wild type and a naturally occurring mutant (G35E) human SF-1 were cloned and fused with green fluorescent protein (GFP). Subcellular distribution pattern of human SF-1 in living cells, whose PKA signaling was either activated or not, was studied by laser confocal microscopy after the validity of the gene sequence was confirmed.Results The transactivation ability of the GFP-SF-1 chimeric protein was highly conserved. Wild type human SF-1 diffused homogeneously within the nuclei of cells when PKA was not active, and converged to clear foci when PKA was activated. Mutant SF-1 diffused within the nuclei even in the presence of PKA activation, surprisingly aggregating as fluorescent dots inside the nucleoli, a phenomenon not altered by PKA.Conclusions Activation of PKA causes wild type, but not mutant SF-1 to alter its subnuclear distribution pattern to a transactivationally active form (foci formation). This finding may throw new light on the mechanism by which PKA activates the orphan nuclear receptor. Background The aim of this study was to identify the subnuclear distribution pattern of human orphan nuclear receptor steroidogenic factor 1 (SF-1) in living cells with and without the activation of protein kinase A (PKA) signal pathway, and thus try to explain the unknown mechanism by which PKA potentiates SF-1 transactivation. Methods Full-length cDNAs of wild type and a naturally occurring mutant (G35E) human SF-1 were cloned and fused with green fluorescent protein (GFP). Subcellular distribution pattern of human SF- 1 in living cells, whose PKA signaling was either activated or not, was studied by laser confocal microscopy after the validity of the gene sequence was confirmed. Results of the transactivation ability of the GFP-SF-1 chimeric protein was highly conserved. Wild type human SF-1 diffused homogeneously within the nuclei of cells when PKA was not active, and converged to clear foci when PKA was activated. Mutant SF-1 diffused within the nuclei even in the presence of PKA ac tivation, surprisingly aggregating as fluorescent dots inside the nucleoli, a phenomenon not altered by PKA. Conclusions Activation of PKA causes wild type, but not mutant SF-1 to alter its subnuclear distribution pattern to a transactivationally active form (foci formation). may throw new light on the mechanism by which PKA activates the orphan nuclear receptor.