Minimal residual disease (MRD), the tumour burden which remains after a course of treatment that has resulted in clinical remission , appears to differ in certain characteristics from the primary tumour population. Certainly the cells which comprise MRD have had to escape from the constraints of the primary tumour mass, invade normal tissue and penetrate small vessels in order to enter the circulation in which they then have had to survive. Such activities are the consequence of the expression of specific proteins and these may well be a reflection of alterations in DNA or RNA levels. Identifying the changes in RNA expression levels between related cell groups exhibiting different phenotypes recently has become a great deal easier as a consequence of developments in analytical procedures such as Differential Display (DD) and Serial Analysis of Gene Expression (SAGE). Application of these procedures to MRD cells recovered from blood, bone marrow or lymph node, should identify novel sequences associated with tumour progression and the development of disseminated disease.
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