Abstract
The X and Y chromosomes of placental and marsupial mammals originated from a pair of autosomes. Ohno proposed that the expression levels of X-linked genes must have been doubled in males to compensate for the degeneration of their Y homologs. Recent mRNA sequencing experiments, however, found at most weak or infrequent X-chromosome dosage compensation. Nonetheless, dosage compensation need not occur at the mRNA level, because ultimately it is the protein concentration that matters. Analyzing human proteomic data from 22 tissues, we here report that X upregulation is absent at the protein level, indicating that Ohno's hypothesis is also invalid at the protein level.