Differential gene expression implies:
Differential gene expression explains how genetically identical cells achieve phenotypic diversity. Every somatic nucleus contains complete genome, yet only subset of genes transcribed in any given cell type due to combinatorial control. Enhancers, promoters, transcription factors such as MyoD in muscle, Ngn3 in pancreas, epigenetic marks including H3K27ac activation and DNA methylation silencing, dictate lineage-specific transcriptomes. Post-transcriptional regulation further refines proteome. This model refutes notion DNA differs among cells or mutations drive specialization, instead emphasizing regulated transcription as driver of cellular identity, tissue function, and developmental progression from zygote to adult.
Ref: Davidson, Genomic Regulatory Systems; Gilbert, 12th ed., Chapter 2: Gene regulation.