The c.639+919G>A mutation, located 4bp upstream of the 3’ splice site of the cryptic exon, increases the recognition of a normally weak splice site, causing a remarkable imbalance in the expression of the two GLA transcripts, with an increase in the alternatively spliced GLA mRNA and a significant reduction of normal spliced mRNA, which would account for the 5–10% residual α-GalA activity detected in patients with the cardiac LO form of FD [23]. The gene discussed is GLA; the disease is Fabry disease.