C1S and Ehlers-Danlos syndrome: Among all subtypes of EDS, periodontal Ehlers–Danlos syndrome (pEDS) (also known as EDS type VIII, OMIM#130080) is a specific EDS subtype caused by autosomal dominant pathogenic variants in complement 1 subunit genes C1R and C1S, with early severe periodontitis as the predominant clinical feature (Kapferer-Seebacher et al., 2016).