Perichromatin fibrils are in situ forms of nascent transcripts.

Ultrastructural investigation of the cell nucleus has so far been the only high-resolution approach for in situ analysis of its structural components and their roles in nuclear functions. Such studies have shown that perichromatin fibrils are the in situ form of hnRNA transcripts. Current evidence strongly supports the idea that they are also sites of pre-mRNA processing steps such as splicing and polyadenylation.