Several models have been presented in the past to explain localized distributions of nuclear RNAs from individual genes that range from small foci to more elongated ''track-like'' structures. We present here a hypothesis which explains that, in the case of regulated splicing, there is in diploid cells a spatial separation of transcription sites from the execution of regulated splicing which we situate to domains of SR protein accumulation, In addition, it explains the presence of poly(A) sequences, and the lack of the autoradiographic label due to short pulses of [H-3]uridine, in these domains. (C) 1996 Academic Press