Aminopeptidase B (Ap-B; EC.3.4.11.6) was originally defined, as an exopeptidase able to trim out basic amino acid residues from the NH₂-terminus of peptides. Although the physiological function of Ap-B remains an open question, several data strongly support the hypothesis that Ap-B could participate in the final stages of precursor processing mechanisms and/or in particular inflammatory processes and tumor developments.

Purifination of Ap-B from rat testes showed that this enzyme is a monomeric 72 kDa Zn²⁺ -dependant exopeptidase, which selectively remove Arg and/or Lys residues from the NH₂-terminus of various peptides. Ap-B is widely distributed in a number of rat and human tissues. Meanwhile, its expression level varies depending of the cells or tissues considered. The enzyme is secreted both by the constitutive and the regulated pathway. Moreover, in PC12 cells, the protein is associated, as an active form, to the external face of their plasma membrane.

Elucidation of the rat, human and mouse Ap-B primary structures allowed its classification in the M1 damily of Zn²⁺ -aminopeptidases and showed a structural relationship with leukotriene A₄ hydrolase, an important enzyme of the arachidonic pathway.

Finally, the human Ap-B gene (rnpep) was localized on chromosome 1 band q32 in a high transcript density chromosomal region. The gene is bracketed by tim17a and elf3, which encode a pre-protein translocase of the inner mitochondrial membrane and an ETS family transcription factor, respectively. The recent description of the mouse genome showed that rnep was localized on chromosome 1 in a putative inversed synthenic region.