Splicing factor 1 is a protein with several isoforms produced by alternative splicing of a single premRNA. These isoforms share a common N-terminal half and have distinct C-terminal portions. In this study, isoforms of SF1 in different cell lines and tissues were identified, and their composition was investigated in different tissues. The results suggest that SF1 isoforms are expressed in a tissue-specific manner. The function of SF1 and its isoforms is not clear. To provide information towards that, we used fluorescence microscopy and demonstrated that SF1 is a new component of paraspeckles. Paraspeckles are formed around non-coding RNAs and although their function is not known, their protein content points to a role in transcriptional control and RNA metabolism. SF1 has been shown previously to bind to a number of ncRNAs. Results from this thesis support the above finding and show that ncRNAs present in paraspeckles associate with SF1.