ABSTRACT The genetic limits of sixty-four deficiencies in the vicinity of the euchromatic-heterochromatic junction of the X chromosome were mapped with respect to a number of proximal recessive lethal mutations. They were also tested for male fertility in combination with three Y chromosomes carrying different amounts of proximal X-chromosome-derived material (BSYy+, y+Ymal126 and y + Ymal +). All deficiencies that did not include the locus of bb and a few that did were male-fertile in all male-viable Df(1)/Dp(1;Y) combinations. Nineteen bb deficiencies fell into six different classes by virtue of their male-fertility phenotypes when combined with the duplicated Y chromosomes. The six categories of deficiencies are consistent with a formalism that invokes three factors or regions at the base of the X, one distal and two proximal to bb, which bind a substance critical for precocious inactivation of the X chromosome in the primary spermatocyte. Free duplications carrying these regions or factors compete for the substance in such a way that, in the presence of such duplications, proximally deficient X chromosomes are unable to command sufficient substance for proper control of X-chromosome gene activity preparatory to spermatogenesis. We conclude that there is no single factor at the base of the X that is required for the fertility of males whose genotype is otherwise normal.