Gene expression can be defined as a group of processes that occur in living cells involved in forming a gene product (proteins or different RNA) from the gene. Gene expression includes steps such as transcription, RNA splicing, translation, and post-translational modification of a protein. It is the basis for cell differentiation: since all cells contain the same genetic information, the expression of different genes leads to different structural and functional cells. In other words, the combination of genes that are turned on and off dictates cellular morphology) and function.
Fertilized single cell quickly divides and new daughter cells began to differentiate (become different from each other) thanks to different gene expression. That’s why the adult cells are way different than the fertilized egg cells.