Therefore, when NO is released from the nitrergic fiber, endothelium or NO donors, diffuses into smooth muscle, and activates GCs, cGMP is produced, but PDE5 does not efficiently degrade it; thus, cGMP accumulates in the smooth muscle cell promoting CC relaxation and excess penile erection in SCD (Musicki and Burnett, 2020). This evidence concerns the gene PDE5A and Schnyder corneal dystrophy.