FOSL1 encodes for 271 amino acid protein with an expected molecular weight of 29.4 kDa. However, in protein extracts isolated from cells stimulated with growth factors or from cancer cells, FOSL1 exhibits multiple bands ranging from 30-40 kDa on immunoblots. Although the appearance of these multiple forms is mainly attributed to post-translational modifications of this protein, FOSL1 exhibits several alternatively spliced variants and one un-spliced form with different transcripts encoding for proteins with different functional domains (http://www.genecards.org/cgi-bin/carddisp.pl?gene=FOSL1). The nature of formation of these alternatively spliced variants of FOSL1 and their functional significance in physiologic and pathologic processes is unclear.

FOSL1 is covalently modified by posttranslational modifications via phosphorylation at serine and threonine residues, mainly through Rsk and ERK1/2/5 MAP kinases.^{[1][2][3][4][5][2][6][7]}These modifications are known to affect both the protein stability and the transactivation potential of FOSL1.^[8] FOSL1 also contains several cysteine residues. The cysteine residues of JUN (a dimeric partner of FOSL1) have been shown to undergo oxidation and glutathionylation as well as sumoylation, which affect its DNA binding activity.^{[9] [10][11]} Whether such modification(s) affects FOSL1 functions, such as its DNA binding activity and transactivation potential, remains to be investigated. FOSL1 contains a nuclear localization signal and is mostly localized in the nucleus. However, immunolocalization studies revealed the presence of a FOSL1 antigen in the cytoplasm in certain situations, such as in the presence of oxidative stress ^[3] and in cancerous tissues.^[12] Although the exact nature of this nuclear-cytoplasmic localization in regulating various cellular processes is unclear, it appears that this trafficking affects the gene expression as well as the stability of this transcription factor.^[3]