2µm thick sections of formalin-fixed, paraffin wax-embedded tissues were stained for immunoglobulin α, γ and µ heavy chains, and λ and κ light chains, CD79a (monoclonal mouse anti-human CD79αcy, clone HM57, DC) and CD3 (monoclonal mouse anti-human CD3, clone F7.2.38, DC) in the presence of controls according to standard or manufacturers' techniques.

Details of epitope retrieval, anti-Ig reagents and controls for immunohistochemical labeling of MRD.

1° antibodies were polyclonal and dilutions were made in Tris buffered saline (10mM, pH 7.5) and sections incubated for 60 minutes. The positive control was reactive cat lymph node. The negative control was mouse anti-human prostate specific antigen antibody (an unrelated antibody), applied to sections in place of the 1o antibody at a dilution of 1/1000. PK: Proteinase K was applied to sections for 5 minutes. Enzymatic proteolytic digestion was halted by application of deionised water. MWPC: Microwaveable pressure cooker (A. Menarini Diagnostics Ltd., Berkshire, UK). Samples were heated for two minutes at full pressure in sodium citrate (10mM, pH6). NO: Nordic Immunological Laboratories, Tilburg, The Netherlands. DC: DakoCytomation Ltd., Ely, UK. SBC-HRP: Streptavidin-biotin complex conjugated with horse radish peroxidase. DAB: 3,3 diaminobenzidine tetrachloride (Sigma Chemicals Co., St. Louis, MO, USA). (Mellor et al 2008).

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	Fig. 1: Immunoglobulin alpha heavy chain - positive staining. A single MRD case expressed IgA, and one case expressed both IgA and IgG (Mellor et al 2008). Unequivocal single light chain immunolabelling (Regezi et al 1983) was used to establish the diagnosis of an MRD in the latter cat. Dual heavy chain labelling has not previously been described in the cat, although it has been reported in human myeloma (Hopper et al 1979, Bakkus et al 2000).
	Fig. 2: Immunoglobulin gamma γ heavy chain - positive staining. A total of 68% of MRD expressed IgG in one study (Mellor et al 2008).
	Fig. 3: Immunoglobulin lambda λ light chain - positive staining. The λ light chain was more commonly expressed by myeloma cells (89%) than the κ light chain in one series (Mellor et al 2008). This is consistent with the physiological distribution of light chains in normal feline plasma cells, where preferential expression of λ is typical (Arun et al 1996).
	Fig. 4: Immunoglobulin kappa κ light chain - positive staining.
	Fig. 5: Immunoglobulin gamma γ heavy chain - positive staining of a giant cell. See photomicrographs and description of giant cells in the “Cytological and histopathological classification of MRD” photomicrographs section.
	Fig. 6: CD79a - positive staining of an MRD. In one study, only 32% of MRD were CD79a positive (Mellor et al 2008). CD79a is expressed from early B-cell progenitors onwards and is variably expressed by human myeloma cells (Grogan 2003).