In mammalian females, the majority of one X chromosome's genes are shut down in a process called X chromosome inactivation. The inactive X is condensed and afterwards called a barrbodies. Because the shutdown is random in the embryonic cells formed by the time inactivation occurs, all mature mammalian females are a inactivate for the expression of X chromosome genes. The inactivation of one X in the female is thought to balance gene expression between males and females, a theory called X-inactivation/Lyonisation. The shutdown of the X chromosome is accomplished by the Xist gene. SRY is the master gene for male sex determination in mammals and is found on the human Y-chromosome.