On getting a T-cell clone and being assured you have one.

Cellular immunology has for years been plagued by the heterogeneity in function and specificity of antigen recognition present within the populations of lymphocytes available for study. The ability to clone single T cells from those populations is therefore a most useful technology because the progeny in any one clone will be of a single functional subtype and, at the very least, will have the great specificity associated with monoclonality. Cloning is being widely used to push forward our understanding of both the antigens that elicit response and the cells that respond. My purpose here, however, is not to review those very exciting applications but rather to address the sometimes thorny issues of the definition of clonality, how it is achieved, and the circumstances in which 'proof' of clonality is or is not essential to the interpretation of data.