An alternative spliced form of FosB is a negative regulator of transcriptional activation and transformation by Fos proteins.

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Abstract
Two forms of FosB transcript and their products can be identified in mouse NIH 3T3 cells following serum induction. The larger RNA codes for a 338-amino acid protein, whereas the smaller RNA results from the removal of an additional 140 nucleotides from FosB mRNA by alternative splicing. This alternative splicing event places a stop codon following the "leucine zipper" region and results in a shorter protein (FosB2) of 237 amino acids that lacks 101 amino acids at the carboxyl terminus. FosB2 is able to form heterodimers with c-Jun and bind to an AP-1 site but is not able to activate the transcription of promoters containing AP-1 sites. Furthermore, FosB2 can not only suppress the transcriptional activation by c-Fos and c-Jun of promoters containing an AP-1 site but also interferes with the transforming potential of viral and cellular Fos proteins. We propose that FosB2 protein functions as a trans-negative regulator.