All proteomes were searched against a local installation of the Pfam database (v.23) [24] using HMMER3 [25] and the script pfam_scan.pl, which was obtained from the Pfam ftp-site (ftp://ftp. sanger.ac.uk/pub/databases/Pfam/Tools/), with Pfam's default settings. In Pfam, the IFITM family is represented by a specific hidden Markov model [Pfam: P04505]. All the proteins that were assigned to this model in the Pfam-search were considered to be homologous to the IFITM family and were therefore included for further analysis. The script pfam_scan.pl uses the homology criterion set by the Pfam database, which is based on a manually curated gathering threshold for each model. The gathering threshold for PF04505 is a score of 20.6. The sequence datasets were controlled for annotated pseudogenes and transcript variants from the same gene. In the case of multiple transcript variants, the longest sequence was kept. The resulting non-redundant datasets were used for the analysis. The bacterial sequences were obtained by querying Uniprot (www.uniprot.org) for the Pfam ID [Pfam: PF04505]. Thereafter, the sequence set was downloaded by browsing by taxonomy and restricting it to species with a full proteome set. To assure that no lineages were missing in the selection of proteomes the nr protein dataset from NCBI was downloaded. The nr datasets contained 15,322,545 (20-09-11) protein sequences from a wide range of organisms. The dataset was searched against the PF04505 Pfam model using HMMER3 with default settings and sequences with a score above the Pfam gathering threshold (20.6) were deemed as homologous to IFITM.