A new study published in Nature Genetics by the Lymphoid Development Group at the MRC Laboratory of Medical Sciences has reveals that the contribution of cells expressing maternal or paternal X chromosomes can be selectively skewed in different parts of the body. The study leverages human data from the 1000 Genomes Project combined with mouse models of human X chromosome-linked DNA sequence variation to advance our fundamental understanding of development in biologically female individuals who have two X chromosomes.