2-Di-1-ASP (DASPI; Compound 18a) is a mono-stryryl dye, and widely used as mitochondrial stain and groove-binding fluorescent probes for double-stranded DNA. 2-Di-1-ASP is selective for G-quadruplex (G4) and double-stranded DNA.In Vitro:2-Di-1-ASP (Compound 18a) displays significant fluorescence enhancements in the presence of G-quadruplex (G4) structures (up to 300-fold), and good selectivity with respect to double-stranded DNA. 2-Di-1-ASP shows fluorimetric selectivity for parallel G4-DNA forms (c-kit2, c-kit87up, c-myc).