* Transcription: The process where the DNA sequence of a gene is copied into a messenger RNA (mRNA) molecule. This mRNA molecule acts as a blueprint for protein synthesis.

* Translation: The process where the mRNA molecule is used to build a protein. Each three-nucleotide "codon" on the mRNA molecule codes for a specific amino acid, which are linked together in a chain to form the protein.

Key points:

* Not all genes are expressed at all times: Gene expression is highly regulated and varies depending on the cell type, developmental stage, and environmental conditions.

* Expression can be controlled at multiple levels: This includes controlling the rate of transcription, the stability of mRNA, and the efficiency of translation.

Therefore, a gene is considered "expressed" if it has been transcribed into RNA and potentially translated into protein. This ultimately leads to the production of a functional product (protein or functional RNA molecule) that influences the cell's function.