Fill in the Blank in the Following Paragraph Regarding X-Chromosome

Fill in the blank in the following paragraph regarding X-chromosome inactivation. Do not use abbreviations.
"The presence of two copies of the X chromosome in females but only one in males poses a potential problem for regulation of gene expression: everything else being equal, the higher copy number of the X-linked genes can lead to an imbalanced overproduction of their products in females compared to males. Organisms have evolved different … mechanisms to solve this problem. In mammals, for example, the almost complete inactivation of one of the X chromosomes provides the solution to the dosage problem."