Figure 1. Surveillance of disease progression and response to therapy using gene expression profiling. Expression profiles were conducted with peripheral blood lymphocyte samples taken from 13 patients at different times. (a) Analysis of filtered expression data for nine patients (arbitrary labels) using singular value decomposition revealed a generic shift in messenger ribonucleic acid expression associated with disease progression, thus defining a progression axis spanned by the first two singular vectors PCA 1 and PCA 2 of the principle component analysis. In order to follow the development of disease in a single patient (‘follow-up patient’), all samples taken from this patient were projected on to the same plane spanned by vectors PCA 1 and PCA 2. Temporally consecutive samples are connected with arrow-marked lines. Thick lines denote the time frame of therapy. (b) Patient (arbitrary labels) with an initial 13q^– deletion acquires an additional 11q^– deletion resulting in progression of the disease. Therapy is able to improve the patient's status. (c) A patient (arbitrary labels) with initially constant disease status receives therapy resulting in improvement. However, during that time he acquires a 17p^– deletion, which results in progression of therapy-resistant disease.