What do proto-oncogenes primarily code for?

Proto-oncogenes primarily code for proteins that promote cell-cycle progression. These proteins are essential for normal cellular functions and regulation of the cell cycle. When functioning properly, proto-oncogenes help to stimulate cell division, ensuring that cells grow and proliferate in a controlled manner.

However, when proto-oncogenes undergo mutations or become overexpressed, they can become oncogenes, leading to uncontrolled cell growth and potentially contributing to cancer development. This transformation highlights the critical balance that proto-oncogenes maintain within normal cellular processes, as they are key regulators of growth signaling pathways.

The other choices refer to functions that are not associated with proto-oncogenes. For instance, proteins that inhibit cell-cycle progression are typically coded by tumor suppressor genes, which serve a different role in regulating cell growth. Likewise, proteins that cause cellular apoptosis are involved in programmed cell death, and those involved in immune response do not fall under the category of proto-oncogenes. Thus, the main role of proto-oncogenes in promoting cell-cycle progression distinguishes them from these other gene functions.