Precursor of the transcription factor form (Processed sterol regulatory element-binding protein 1), which is embedded in the endoplasmic reticulum membrane. Low sterol concentrations promote processing of this form, releasing the transcription factor form that translocates into the nucleus and activates transcription of genes involved in cholesterol biosynthesis and lipid homeostasis.; Key transcription factor that regulates expression of genes involved in cholesterol biosynthesis and lipid homeostasis. Binds to the sterol regulatory element 1 (SRE-1) (5'-ATCACCCCAC-3'). Has dual sequence specificity binding to both an E-box motif (5'-ATCACGTGA-3') and to SRE-1 (5'-ATCACCCCAC-3'). Regulates the promoters of genes involved in cholesterol biosynthesis and the LDL receptor (LDLR) pathway of sterol regulation.; Isoform expressed only in select tissues, which has higher transcriptional activity compared to SREBP-1C. Able to stimulate both lipogenic and cholesterogenic gene expression. Has a role in the nutritional regulation of fatty acids and triglycerides in lipogenic organs such as the liver. Required for innate immune response in macrophages by regulating lipid metabolism.; Predominant isoform expressed in most tissues, which has weaker transcriptional activity compared to isoform SREBP-1A. Primarily controls expression of lipogenic gene. Strongly activates global lipid synthesis in rapidly growing cells.