Immunoglobulins from different species share similar structures. Secondary antibodies raised against one species are likely to recognize epitopes on other species’ immunoglobulins through those shared structures. This cross-reactivity can cause background tissue staining or signal from an non-target primary antibody. When experiments require the use of material from multiple species, secondary antibodies with minimal cross-reactivity (min X) to serum proteins from non-target species are an indispensable component to the immunoassay toolbox.