Every colorectal cancer (CRC) patient should be tested for Microsatellite Instability (MSI), as a first screen for Lynch syndrome (LS). Recently, clinical benefit from immune checkpoint blockade was reported in metastatic MSI CRCs. Hence, there is a high level of interest in studying the MSI phenotype as a potential prognostic and predictive biomarker in CRC patients. In the present work, we demonstrate that a new marker, HT17, simplifies and improves the current standard molecular methods for detecting MSI in colon tumor samples. We believe there is a strong rationale to use HT17 when screening for MSI in CRC. (http://jmg.bmj.com/content/early/2016/02/01/jmedgenet-2015-103518 )