Abstract: A method for detecting aggressive tumor behavior and/or increased risk for tumor metastasis generally includes analyzing a tumor sample from the subject for expression of transcripts from coding regions of the cell cycle gene cluster, the immune-1 gene cluster, or the immune-2 gene cluster; computing a sum of log2-transformed mean-centered expression values, thereby generating a Gene Cluster Expression Summary Score (GCESS) for the sample; and detecting a tumor with aggressive behavior and/or increased risk for tumor metastasis. An aggressive tumor and/or increased risk for tumor metastasis may be indicated where the cell cycle gene cluster is analyzed and the sample GCESS is greater than 0, or the immune-1 gene cluster or the immune-2 gene cluster is analyzed and the sample GCESS is less than 0.