Cancer is composed of multiple cell populations with different genomes. Each of the populations is called a clone (or subclone) and this phenomenon is called intratumor heterogeneity (ITH). ITH is observed in various types of cancers and presumed to be a major cause leading to therapeutic resistance. If a tumor harbors a major clone sensitive to a specific anti-cancer treatment, the tumor shrinks within a given period after the treatment. However, in most cases, a minor clone resistant to the chemotherapy exists in the tumor and predominantly regrows in spite of the intensive therapy. It is supposed that ITH can be generated by clonal branching during cancer evolution. This article is protected by copyright. All rights reserved.