Some studies have claimed that biological factors may be largely responsible for the disproportionate success in sprinting events enjoyed by athletes of West African descent. Chief among these is a preponderance of natural fast twitch muscle fibers, which aid in quicker reaction times. Scientists have concluded that elite-level sprinting is virtually impossible in the absence of the ACTN3 protein, a "speed gene" most common among persons of West African descent that renders fast twitch muscle fibers fast.[12] Top sprinters of differing ancestry, such as Christophe Lemaitre, are believed to be exceptions in that they too likely have the genes favourable for sprinting.