Fig. 1. Phylogenetic tree of immune system–related studies in mammal orders from 2009 to 2019.

Metastatic cancer has been reported in nearly all mammalian orders, and MHCs have been the most intensely studied molecules in most orders. In the past decade, studies of immune checkpoint molecules (PD1, PDL1, and CTLA4) have become a primary focus in humans and rodents. However, immune checkpoint studies in other species are limited, particularly at the protein level, because of the lack of species-specific reagents. This creates a vast gap in our understanding of the evolution of the mammalian immune system. The numbers in the columns represent the number studies matching Web of Science search results between 2009 and 2019. See table S1 for search terms.