After its first description in 1924, and naming by Evelyn Howard in 1939 to reflect its transient and unknown function, the X-zone of the mouse adrenal attracted a great deal of attention from endocrinologists. What factors were responsible for its appearance at the age of 812 days, growth, and then disappearance with puberty in the male and first pregnancy in the female? A great deal was discovered, even given the limited techniques available, about which hormones control, directly or indirectly, the X-zone. Its function, however, remained unknown with the possible production of steroid hormones apparently eliminated. Having worked briefly on the Z-zone in 1965my only foray into the adrenalI took the opportunity of retirement to look up what had happened in research on the X-zone. Had the enigma of its function been solved? I was surprised that although seemingly isolated bits of research had added to knowledge of systemic and putative local factors controlling the life and death of the cells, and an enzyme concerned with steroid catabolism had been found in the zone, knowledge of the X-zone has not progressed to any significant extent since the 1980s. We still do not know what it does. Mice are not the only mammals to have seemingly special cells forming unusual zones in their adrenals. I will, therefore, point out the importance of a comparative approach while describing the historical evidence on X-zone control and function, including relations between the adrenal medulla and cortex, while stressing the importance of understanding how whole organsand whole animalswork. The X-zone remains Zone X.