Overview of repeats
A polyalanine repeat expansions in exon 1 of the HOXA13 gene has been described, causing the hand-foot-genital syndrome (HFG) (Mortlock et al., 2000). In exon 1 there are three alanine tracts, with a length in healthy humans corresponding to either 14 (HOXA13_1), 12 (HOXA13_2) or 18 (HOXA13_3) trinucleotide repeats (Mortlock et al., 2000).
In the first tract (HOXA13_1), an eight repeat expansion has been detected in affected individuals of one family who carry an allele with 22 polyalanines, but not in the unaffected daughter (Innis et al., 2004). Pathogenic phenotype has been shown in patients who had six additional alanines inserted into the second tract (HOXA13_2), resulting in an 18 repeat long allele (Frisen, 2003). Finally, allele expansions in the third, largest tract (HOXA13_3) has been found to be expanded from 18 repeats to 24 (Utsch et al., 2002), 26 (Goodman et al., 2000) or even to 29 and 30 repeats of alanines (Innis et al., 2004).