Which of the following steps in gene expression does regulation occur?

Regulation of gene expression can occur at multiple points in the gene expression process, including initiation, translation, and post-translational modification.

During the initiation of transcription, regulatory proteins can enhance or inhibit the binding of RNA polymerase to DNA, determining whether a gene will be transcribed into mRNA. This represents a critical control point for gene expression.

Translation of mRNA is also subject to regulation. Specific factors can influence the ribosome's ability to bind to mRNA and initiate protein synthesis, thus determining how much protein is produced from the mRNA. Regulatory elements in the mRNA itself, such as untranslated regions (UTRs), can also play a role in this process.

Post-translational modification adds another layer of regulation after the protein has been synthesized. Proteins can undergo various modifications, such as phosphorylation, glycosylation, or cleavage, which can alter their function, activity, location, and stability within the cell.

Since all these processes – transcription initiation, translation, and post-translational modification – are levels at which gene expression can be regulated, the correct answer reflects the comprehensive nature of gene expression control.