GLUT6 is expressed in various tissues in the body, but doesn't encode any functional glucose transport protein (1). Why does it occur at all? It's thought that GLUT6’s nucleotide sequence may have simply occurred from an insertion of GLUT3 reverse transcribed in a region of an untranslated gene (1).
1. Kayano T, Burant CF, Fukumoto H et al. Human facilitative glucose transporters. Isolation, functional characterization, and gene localization of cDNAs encoding an isoform (GLUT5) expressed in small intestine, kidney, muscle, and adipose tissue and an unusual glucose transporter pseudogene-like sequence (GLUT6). J Biol Chem 1990;265:13276-82.