The distinction between certain benign cutaneous lymphocytic infiltrates and cutaneous T-cell lymphoma (CTCL) can be a challenging problem in clinical practice. An imperfect but useful tool in this circumstance is the analysis of T-cell clonality or monoclonality by assessing T-cell receptor (TCR) gene rearrangements. "Monoclonality" describes the origin of a specific human malignant tumor from one single cell from which the entire tumor is derived. The term is used to describe the early steps in tumorigenesis; in later stages of tumor growth and progression, monoclonal tumor cells may acquire a mutator phenotype, rendering the genome unstable. The resulting genetic heterogeneity may ultimately lead to subclones in the formerly monoclonal population of tumor cells.