Sodium Fluoride (Fluoride, NaF) is a commonly used general inhibitor for protein phosphoseryl and phosphothreonyl phosphatases (PSPs). Routinely Fluoride, combined with Sodium Orthovanadate (NEB# P0758), a general inhibitor for protein phosphotyrosyl phosphatases (PTPs), is used to preserve the protein phosphorylation state in cells, cell lysates, and protein kinase (PK) assays. These inhibitors should ideally be present in all buffers used in the purification of PKs (1, 2).