Feline coronaviruses (FCoV) exist as 2 biotypes: feline enteric coronavirus 
(FECV) and feline infectious peritonitis virus (FIPV). FECV causes subclinical 
infections; FIPV causes feline infectious peritonitis (FIP), a systemic and 
fatal disease. It is thought that mutations in FECV enable infection of 
macrophages, causing FIP. However, the molecular basis for this biotype switch 
is unknown. We examined a furin cleavage site in the region between 
receptor-binding (S1) and fusion (S2) domains of the spike of serotype 1 FCoV. 
FECV sequences were compared with FIPV sequences. All FECVs had a conserved 
furin cleavage motif. For FIPV, there was a correlation with the disease and >1 
substitution in the S1/S2 motif. Fluorogenic peptide assays confirmed that the 
substitutions modulate furin cleavage. We document a functionally relevant S1/S2 
mutation that arises when FIP develops in a cat. These insights into FIP 
pathogenesis may be useful in development of diagnostic, prevention, and 
treatment measures against coronaviruses.