The BPIFB5 protein and Bpifb5 gene have been characterized in mammals such as rodents (mouse,[3] rat[4]) and even-toed ungulates (pig,[5] cow[6]) but are apparently lacking in primates and other vertebrates such as birds, reptiles, and amphibians.

[7] The protein in rodents is expressed at moderately high levels in mucosa of the airways (respiratory and olfactory epithelium) and at moderate levels in salivary glands, esophagus, and gonads (ovary, testis);[8] in even-toed ungulates expression is high in testis, moderate in brain and striated muscle, and low in kidney.

[14] The BPI fold creates apolar binding pockets that can interact with hydrophobic and amphipathic molecules, such as the acyl carbon chains of lipopolysaccharide found on Gram-negative bacteria, but members of this family may have many other functions.

The human BPIFB5P pseudogene is found clustered with other members of the BPIF gene family conforming to the pattern observed in mammals, but this is not the case for other vertebrate species.

In a systematic analysis of the chicken genome, the Lplunc1(Bpifb1) / Lplunc5(Bpifb5) branch of the gene family was determined to be absent, therefore BPIFB1 and BPIFB5 proteins likely arose only after the speciation of mammals.