**1. Introduction**

Canine transmissible venereal tumor (CTVT) is the oldest known contagious cancer in dogs in the world. The first mention of this cancer occurred in the nineteenth century. It is also known as canine infectious sarcoma, canine venereal granuloma, canine transmissible lymphosarcoma, canine round cell sarcoma, and canine Sticker's sarcoma [1, 2]. CTVT has an etiology like that of other contagious cancers, such as devil facial tumor disease (DFTD), which originates from an abnormal cell line with an unlimited proliferative capacity [3, 4]. CTVT can be transplanted via viable cancer cells that naturally allograft between CTVT-infected dogs and uninfected hosts via physical transfer [4].

Previous studies suggest that CTVT cell lineage might be up to 10,000 years old [5]. The clonal origin of CTVT was proven through the analysis of microsatellite polymorphisms, mitochondria DNA (mtDNA), dog leukocyte antigen (DLA) typing, and genome sequencing. It has been suggested by phylogenetic analysis that CTVT emerged between 4000 and 8500 years ago in Asia [6–9]. CTVT is now a common disease worldwide that has been reported on all inhabited continents. However, CTVT has a higher prevalence in tropical and subtropical regions and is uncommon

in North America and Northern and Central Europe, although occasional cases have been reported in imported dogs [5, 10]. An interesting feature of this cancer is that it is usually curable via several protocols. However, metastasis, chemotherapeutic resistance, and death are still reported in CTVT cases in endemic areas, especially in immunosuppressed dogs [1, 11].
