We provide evidence that the four IFITM genes together with ten additional human genes, known as TUSC5, TMEM233, PRRT2, TMEM90A, DSPC2, TMEM90B, TMEM91, AC023157, AL160276 and AC068580, form a novel gene family that we call the Dispanins, which refers to the 2TM membrane topology that is common to all identified members. This family is the second largest 2TM family in the human genome, superseded only by the Inwardly rectifying potassium channel family that has 15 members [2] . Except for the 2TM memebrane topology the Dispanins are not homologous or share domains with any other 2TM proteins in the human genome and constitute a distinct gene family. We have discovered that this family is found in metazoan, the choanoflagellate M. brevicollis and the brown alga E. siliculosus, but not in other eukaryotes. Surprisingly it is widely present in bacteria where it is found in several different phyla such as Actinobacteridae, Acidobacteria, Cyanobacteria, Bacteriodetes, Firmicutes and Proteobacteria. The highest number of bacterial Dispanins is detected in Actinobacteria and Proteobacteria, which diverged around three billion years ago [15] . We find that Dispanins in eukaryotes and bacteria have high sequence similarities and share several conserved sequence motifs (Figure 4) , which is strong evidence for a common evolutionary origin and possibly a functional relationship. As the family is found in several bacterial phyla we suggest that it first emerged in bacteria to later be introduced in eukaryotes through a horizontal gene transfer event. However, we were not able to construct a stable phylogenetic tree including bacterial and eukaryotic Dispanins. As the eukaryotic Dispanins only is widespread in metazoa it was unexpected to find the family in the evolutionary distant brown alga E. siliculosus (Figure 2 ). Our sequence analysis supports that all metazoan Dispanins have their origin in the common ancestor of M. brevicollis and metazoa as the choanoflagellate share a conserved splice site in the intracellular loop with nearly all metazoan Dispanins (Figure 4) . However, the finding of the family in E. siliculosus suggests that the family has undergone two horizontal gene transfer events. As the E. siliculosus Dispanins are more similar to metazoan family members than M. brevicollis and bacteria we propose that the first horizontal gene transfer event was from bacteria to a common ancestor of choanoflagellates and metazoa followed by a second transfer between metazoa and brown alga.