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Clonality in Langerhans' cell histiocytosis

Most human tumours are monoclonal, which suggests that they originate from a single cell. “Clonality” can be investigated by several techniques. In plasma a dominant monoclonal class of immunoglobulin suggests a neoplasm derived from a single altered plasma cell, which gives it a survival and growth advantage over non-neoplastic cells. In lymphoid malignancies, studies on rearrangements of genes coding for antigen receptors provide an excellent system of clonal markers, with consistent rearrangement of clonal immunoglobulin heavy chains (in B cell leukaemias and lymphomas)1 and T cell receptor s chains (in T cell tumours). In other tumours careful cytogenetic and molecular analysis has shown consistent reciprocal chromosomal translocations and partial and complete chromosomal deletions or duplications which reflect tumour clonality as well as indicating stretches of altered sequences of DNA that may have a role in the initial neoplastic change. Cytogenetic and DNA analysis has not yet, however, provided clonal markers for many solid tumours.2

An alternative approach, applicable only to females, is to take advantage of the phenomenon of “Lyonisation” (X inactivation mosaicism).3 4 Early in …