In 2018, Hatch et al examined PD-1 and PD-L1 by IHC in 55 tissues from patients with mastocytosis, demonstrating a PD-L1 expression in 77% of bone marrow biopsies and 92% of skin biopsies including all mastocytosis types; they found no expression of PD-1 or PD-L1 within mast cells in healthy or reactive bone marrows or cases of myelodysplastic syndrome or myeloproliferative neoplasms [16]. Here, CD274 is linked to mastocytosis.