**Preface to "Molecular Mechanisms of Steroid Hormone Biosynthesis and Action"**

Dear Colleagues,

Steroids are essential hormones that regulate biological processes across the lifespan. Steroid hormone production and action must be tightly controlled otherwise there can be detrimental effects on physiological function leading to disease.

The biosynthesis of steroids and their actions in target tissues and cells are complex but coordinated processes that rely on a series of diverse cellular and molecular events that include proper steroidogenic cell differentiation and gene expression, hormone transport, hormone processing, and hormone action via interaction with specific receptors. The regulation of these events requires the concerted action of various hormones, growth factors, transcription factors, and other signaling and regulatory molecules to ensure adequate genomic, cellular and/or physiological responses. Many of these processes are also targets for endocrine disruption.

This reprint contains original (15) and review (4) articles by experts at the cutting edge of their fields published in a Special Issue on the "Molecular Mechanisms of Steroid Hormone Biosynthesis and Action" of the *International Journal of Molecular Sciences*. The topics are broad and include: 1) Steroid hormone biosynthesis, including substrate availability, mechanism of steroidogenic enzyme action and impact of mutations; 2) Molecular and cellular regulation of steroidogenesis, including steroidogenic cell response to hormone stimulation, signaling pathways, kinases, transcription factors, and gene expression; 3) Molecular mechanisms of endocrine disruptor action on steroidogenic cells; and 4) New tools and approaches to detect and study steroid hormones.

I sincerely hope that the readers will appreciate the breath of information this reprint contains in addition to encouraging them to further explore this constantly evolving and fascinating field.

Finally, I would like to express my sincere gratitude to all the authors and referees for their dedicated contribution to this Special Issue.

> **Jacques J. Tremblay** *Editor*
