Early detection of cancer increases the probability of recovery. This paper presents an intelligent decision support system (IDSS) for the early diagnosis of cancer based on gene expression profiles collected using DNA microarrays. Such datasets pose a challenge because of the small number of samples (no more than a few hundred) relative to the large number of genes (in the order of thousands). Therefore, a method of reducing the number of features (genes) that are not relevant to the disease of interest is necessary to avoid overfitting. The proposed methodology uses the information gain (IG) to select the most important features from the input patterns. Then, the selected features (genes) are reduced by applying the grey wolf optimization (GWO) algorithm. Finally, the methodology employs a support vector machine (SVM) classifier for cancer type classification. The proposed methodology was applied to two datasets (Breast and Colon) and was evaluated based on its classification accuracy, which is the most important performance measure in disease diagnosis. The experimental results indicate that the proposed methodology is able to enhance the stability of the classification accuracy as well as the feature selection.Symmetry 2020, 12, 408 2 of 15 the age of 20 in the United States. After leukaemia, brain cancer and others, tumours of the CNS are the second most common type of cancer among children; the rate of such tumours has never reached more than 26% among children under the age of one year [1,2]. In 2019, 1,762,450 new cancer cases of brain and other nervous system tumours were reported in the United States, and the number of associated deaths was estimated to be 606,880. Thus, it is important to develop a methodology of detecting cancer in the early stages before the tumour worsens, thereby reducing the risk of death [3].The conventional methods of diagnosing most existing diseases depend on human experience to recognize cases that correspond to confirmed data patterns. However, this age-old diagnosis methodology is subject to human error and imprecise diagnosis and is both time-consuming and labour-intensive, thus causing undue stress throughout the whole process. As an alternative, computer-aided diagnosis (CAD) systems based on machine learning have been continually improving and are employed to support specialists in the determination of diagnosis decisions [3][4][5].Most current CAD systems for medical diagnosis depend on diverse information, such as medical laboratory tests (e.g., blood tests and magnetic resonance imaging (MRI)), medical indicators (finger tremors and lung signs or symptoms), and various types of digital images (such as X-rays and ultrasound images). However, physical medical examinations pose a risk of transmission of infection through tools and other channels, such as scratching of the skin while taking a blood sample [6][7][8]. X-rays are harmful because of the exposure of body cells to radiation. The quality of ultrasound data depends on the accuracy and integrity of the image, whi...