Vitamin C

The loss of activity of the gene for L-gulonolactone oxidase (GULO) has occurred separately in the history of several species. GULO activity has been lost in some species of bats, but others retain it. The loss of this enzyme activity is responsible for the inability of guinea pigs to enzymatically synthesize vitamin C. Both these events happened independently of the loss in the haplorrhini suborder of primates, including humans.

The remains of this non-functional gene with many mutations is, however, still present in the genomes of guinea pigs and humans. It is unknown if remains of the gene exist in the bats who lack GULO activity. The function of GULO appears to have been lost several times, and possibly re-acquired, in several lines of passerine birds, where ability to make vitamin C varies from species to species.

- Wikipedia

So if GULO is defective in more than one unrelated species, it cant be used to determine with any conclusiveness that two species are related.

The evolutionist will counter that the great apes (humans included) are all missing the exact same exons within the GULO gene, when compared to the guinea pig and flying fox.  

"Exons shown in white with dotted borders are missing altogether. Exons shown in green have been matched with a high degree of confidence. Exons shown in orange are highly mutated and have only been matched with a low degree of confidence."