Four decades of Hox gene investigation and many more to go.

Forty years ago, Ed Lewis established for the first time the organization of homeotic genes along the chromosome and its importance in embryo patterning. To celebrate this seminal discovery, the International Journal of Developmental Biology decided to launch a Special Issue. It is with honor, pleasure, but also humility that we accepted the challenge of acting as guest editors for this Special Issue. We entitled the issue Hox genes: past, present and future of master regulator genes since despite four decades of amazing discoveries, numerous questions remain unanswered, which open new avenues of research. This is well-acknowledged by Robb Krumlauf and Jacqueline Deschamps in the Introductory articles. The high-level reviews and original research reports collected in this Special Issue also reflect the wide-range and important topics that are still in the spotlights including the origins of Hox genes, the regulatory events controlling their expression, the mechanisms driving the action of HOX proteins, and their multiple roles in normal development and pathogenesis.