The inducibility of tyrosine aminotransferase (EC 2.6.1.5) by corticosteroid hormones in rat-human hybrid clones was studied. The presence of human X chromosome activity in the cells was always associated with the suppression of tyrosine aminotransferase inducibility in all the clones examined. Negative correlation between the human X chromosome and inducibility of the enzyme was clearly established. Corticosteroid receptor was present to the same extent in hybrid cell clones that either contained or lost the human X chromosome. The human repressor for inducible tyrosine aminotransferase has a linkage relationship with glucose-6-phosphate dehydrogenase (EC 1.1.1.49) and hypoxanthine-guanine-phosphoribosyltransferase (EC 2.4.2.8) and, therefore, can be assigned to the X chromosome.