Steroids are widely dispersed throughout the animal kingdom where they act as regulators of numerous biochemical and physiological processes.
In vertebrates, the nuclear receptor superfamily contains receptor proteins with specific affinity to estrogens, androgens, progestins, glucocorticoids, mineralocorticoids and thyroid hormones. Besides of them, estrogenrelated receptor proteins (ERRs) and the ecdysone receptors of the invertebrate ecdysozoans belong to the nuclear receptor superfamily.
Three of the above-mentioned ligand groups (estrogens, androgens, and progestins) are usually addressed by the term ‘‘sex steroids’’ since they are involved in sex-specific regulation of molecular processes. In vertebrate reproductive endocrinology estrogens play a pivotal role via binding to estrogen receptors (ER). In invertebrates, however, the origin and relevance of estrogens remains unclear. It is believed that ER orthologs are lost within the ecdysozoan clade since, within protostomes, ER receptor genes and proteins have only been found in molluscs. But, evidence exists from environmental regulations and testing programs that endocrine-disrupting industrial chemicals with estrogenic-activity (xenoestrogens) affect the development and differentiation of some insects and crustaceans.