Are all DNA molecules protein-coding?

No! Protein-coding DNA, which can be transcribed into mRNA that will be further translated into protein, makes up barely 2% of the human genome; more than 98% of DNA molecules are noncoding. Although non-coding DNA do not provide instructions for making proteins, they may encode for non-coding RNA such as microRNA, ribosomal RNA (rRNA), transfer RNA (tRNA), or enzymatic RNA molecules called ribozymes. In addition, non-coding DNA plays important roles in regulate cell functions, especially the control of gene activity.