PRIOR GENE EXPANSION. “Classes of transcription factors involved in animal development, such as HOX, SOX, POU, F-box, and T-box, emerged most likely before the divergence of the different bilaterian animals but after the split of the basal choanoflagellate lineage from the bilaterian lineage. These metazoan transcription factor classes greatly expanded through duplication in the bilaterian ancestor, just before the Cambrian “explosion”.”
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