However, one conundrum is why so few GLRB mutations are found in startle disease relative to GLRA1, or SLC6A5, encoding the presynaptic glycine transporter GlyT2, which are the primary and secondary genetic causes of startle disease respectively (Carta et al., 2012; Giménez et al., 2012; Rees et al., 2006). This evidence concerns the gene SLC6A5 and hereditary hyperekplexia.