Nineteen strains (129X1/Sv, 129P2/OlaHsd, 129S1/SvImJ, 129S5SvEvBrd, AKR/J, A/J, BALB/cJ, C3H/HeH, C3H/HeJ, CBA/J, DBA/1J, DBA/2J, LP/J, NZO/HILtJ, RF/J, SEA/GnJ, FVB/NCrl, FVB/NJ, BUB/BnJ, ZALENDE/EiJ, LEWES/EiJ and WSB/EiJ) contain an in-frame 12 bp deletion within the gene spanning the basepairs 424 to 435. Immunocytochemistry analysis confirmed the absence of protein expression in vomeronasal sensory epithelium and neutrophil granulocytes. Furthermore, HEK293T cells expressing Fpr3del showed missing receptor protein expression and loss of function. This deletion is not detected in 13 other strains (C57Bl/6NCrl, C57Bl/6NJ, C57Bl/10J, C57BR/cdJ, C57L/J, C58/J, I/LnJ, NZB/B1NJ, NZB/Ola, NZW/LacJ, KK/HiJa, NOD/ShiLtJ, ST/bJ, SPRET/EiJ, CZE/DT, PWK/PhJ, GER/DT, FRA/DT, CAST/EiJ and KAZ/DT).