Abstract: The present disclosure relates to peptides and proteins which may be used to modulate cell proliferation or inhibit infection. It is based, at least in part, on the discovery of peptides and proteins isolated from embryonic tissue (developmental peptides and proteins, or “DPs”), certain of which have been found to exhibit an antiproliferative effect on a variety of cancer cells and/or to act as broad-spectrum antiviral agents, and others of which conversely promote cell proliferation. It is further based on the discovery that DPs modulate phosphorylation of certain proteins associated with normal and/or aberrant cell proliferation.