The present exploratory study, focused on the C26 variant of cystatin D, which is able to participate with multiprotein complexes’ (mPCs) formation [21] and it is not detectable with a top-down proteomic approach, demonstrated not only that salivary cystatin D-C26 interactome exhibited peculiar protein composition in healthy subjects (Ctrls) and in patients affected by mastocytosis, but also the different protein composition between the two subgroups of patients affected by systemic mastocytosis with and without cutaneous symptoms. The gene discussed is CST5; the disease is mastocytosis.