These genes play a part in the normal functioning of lysosomes which are involved in the breakdown of cellular waste materials. It hasn't been discovered why damage to these genes specifically affect speech, but it causes stuttering. 

University of Rochester

As with language acquisition, children learn to produce languages more easily than adults. There is also a genetic connection in language production as with language acquisition. Children born with Williams syndrome have unusually excelling verbal skills. However, some genetic abnormalities can impair speech production and cause stuttering. 

Language production

Breedlove, S. & Watson, N. (2017). Behavioral neuroscience, eighth edition. Oxford University Press.

Behavioral Neuroscience 8th Edition

GNPTAB, GNPTG, and NAGPA genes

A disorder characterized by fluent linguistic function but poor performance on standard IQ tests and great difficulty with spatial processing. This syndrome is caused by the absence of about 28 genes from one of the two chromosomes numbered 7.

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