A principal component analysis of the TCGA data for 15 cancer localizations
unveils the following qualitative facts about tumors: 1) The state of a tissue
in gene expression space may be described by a few variables. In particular,
there is a single variable describing the progression from a normal tissue to a
tumor. 2) Each cancer localization is characterized by a gene expression
profile, in which genes have specific weights in the definition of the cancer
state. There are no less than 2500 differentially-expressed genes, which lead
to power-like tails in the expression distribution functions. 3) Tumors in
different localizations share hundreds or even thousands of differentially
expressed genes. There are 6 genes common to the 15 studied tumor
localizations. 4) The tumor region is a kind of attractor. Tumors in advanced
stages converge to this region independently of patient age or genetic
variability. 5) There is a landscape of cancer in gene expression space with an
approximate border separating normal tissues from tumors