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The authors have studied the interaction between the abnormal oocyte mutation and an inversion of the X chromosome, In( 1)sc(4), which has a proximal breakpoint in or near the heterochromatic region (ABO) that maternally interacts with the abo product. It has been demonstrated that the presence of X chromosomes carrying this inversion, besides a marked increase in the severity of the maternal effect of the abo mutation, produces a zygotic effect resulting in the lethality of the progeny of stocks homozygous for abo and sc(4). These results indicate that the sc(4) inversion carries an abnormal region indispensable for the development of abo zygotes from sc(4);abo mothers.