Factor required for mammalian spliceosome assembly is localized to discrete regions in the nucleus.

A monoclonal antibody raised against mammalian spliceosomes specifically recognizes a non-snRNP factor required for spliceosome assembly. This splicing factor is highly concentrated in discrete regions within the nucleus, in a pattern that is a distinct subset of that seen with anti-snRNP antibodies. These observations are evidence that spliceosome assembly could be compartmentalized within the nucleus.