B2M and head and neck squamous cell carcinoma: The decreased level or total loss of immunoreactive staining for β2M may be due to the loss of the β2M locus, which has been shown to occur in early stages of lymph node-positive metastasising HNSCC lesions (Bockmuhl et al, 2002); mutations in the coding region of β2M, leading to a decrease or loss of β2M expression; or mutations in or methylation of the introns or promoter of β2M, leading to a decrease or total loss of expression (Feenstra et al, 1999a, 1999b; Koene et al, 2004).