Transcription factors (TFs) are proteinsthat typically bind DNA and affect transcription in a positive or negative fashion.While several TF families exist based onprotein structure, they can be categorizedas general or differential. General (orbasal) TFs, such as TATA binding protein,are ubiquitously expressed, and requiredfor the expression of a battery of genesregardless of cell type. In contrast, differential TFs are limited spatially andtemporally, and dictate cell speci?c transcription in response to developmental orenvironmental cues. In this issue, Millsand Taghert suggest a novel category,termed scaling factors, for a group of TFsthat are expressed exclusively in maturecells, dictate the expansion of a particularsubcellular organelle or resource, controlquantitative features of the cell and persist for life. The concept suggests thatscaling factors enhance, rather thaninitiate, existing transcriptional activity,thereby contributing to expansion ofspeci?c cell compartments [1].
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