Model of the Origin of the Naip Gene Array in 129. The essential features of this model are as follows. First, the single ancestral Naip gene became duplicated. This duplication may have occurred due to an unequal crossing over event between different copies of an interspersed repetitive element. This original duplication event is strongly suggested by the sequence similarity profiles between different Naip genes and represents the ancient split between the Naip1/2/3 and the Naip4/5/6/7 families. Second, the proto Naip4/5/6/7 locus becomes flanked by Naip2on its centromere proximal side and by Naip1 andNaip3 on its centromere distal side. The mechanisms whereby this occurred are obscure, but the possibilities include additional duplications of the array via unequal crossing over and deletion or gene conversion of some of the resulting distal loci. Third, the origin of the central portion of the Naip array, including the ΔNaip loci, occurred much more recently; a model for this is described elsewhere (Growney et al. 2000).