A typical human cell expresses only about 5% of its genes at any time. The expression of a gene can be regulated during any step in the transition from a gene to a protein. In each step, there is a control point where the gene can be turned on or turned off. The gene expression can direct embryonic development during cellular differentiation in an embryo. The proteins responsible for the regulation are called

A) enzymes.
B) cytoplasmic.
C) nucleic acids.
D) ribonucleic acids.