Which of the following best describes exons?

Exons are specific sequences within a gene that are retained in the mRNA after the transcription and processing stages. When a gene is transcribed, it initially produces a precursor mRNA (pre-mRNA) that contains both exons and introns. During RNA processing, the introns, which are non-coding sequences, are removed through a process called splicing, while the exons, which consist of coding sequences, are joined together. The resulting mature mRNA only includes these exons, which will be translated into a protein.

This understanding highlights the crucial role of exons in the expression of genes, as they directly contribute to the coding of proteins that perform various functions within the cell. The other options describe different aspects of molecular biology, but they do not accurately define exons. For instance, non-coding sequences refer to introns, regulatory sequences pertain to elements that influence the transcription process rather than coding for proteins themselves, and proteins made from ribosomes result from the translation of mRNA and are not directly a characteristic of exons.