When an organism is under stress the organism's homeostatic balance is upset. Factors in the organism's internal or external environment such as temperature and light intensity can influence a set of genes called regulatory genes by the release of hormones and growth factors which could then influence the growth and development of the organism.
Retinoic acid, a derivative of vitamin A, and drugs like thalidomide are teratogenic or morphogens. They can influence development. Thalidomide acts on a Hox gene to prevent proper limb formation during pregnancy. Retinoic acid activates Hox genes and can lead to cranial deformities.
Thalidomide is now used to prevent the formation of the network of capillaries needed for tumours to grow (angiogenesis).