Peptides and steroids differ extremely in chemical structure, but also in a way they function. Thus, steroid hormones bind to the cell surface where they act on the cell receptors (kind of cellular switches), thus making various changes in the cells. However, peptide hormones enter the cells and bind directly to the cellular DNA, and thus they change the DNA behavior or its transcription. Sexual hormones like testosterone and estrogen are examples of steroids, and calcitonin and human growth hormone (HGH) are examples of peptides.