It has been postulated that, during evolution of the Ig superfamily, modifications of the function of individual receptors might occur by acquisition of exons and their subsequent modification, though evidence of this is lacking. Here we have analysed the interaction of mouse IgG subclasses with high‐affinity FcγRI (CD64) which contains three Ig‐like domains and is important in innate and adaptive immunity. This analysis has identified a mechanism by which the postulated modification of newly acquired exons provides gains in function. Thus, the most widely distributed FcγRI allele in mice (eg BALB/c), bound only a single IgG subclass, IgG2a, with high affinity. However, non‐obese diabetic (NOD) mice expressed a unique allele that exhibits broader specificity and, in addition to binding IgG2a, FcγRI‐NOD bound monomeric IgG3 and bound IgG2b with high affinity, an IgG subclass not bound by FcγRI of other mouse strains, either as monomer or multivalent immune complexes. Analysis of mutants of FcγRI wherein segments of the interdomain junctions were exchanged between FcγRI‐BALB and FcγRI‐NOD identified these regions as having major influence in ‘gain‐of‐function’by the NOD form of FcγRI. Nucleotide sequence analysis of intron/exon boundaries encoding the interdomain junctions of the FcγRI alleles showed these to have arisen by mutation to alter existing or create new mRNA splice donor/acceptor sites, resulting in generation of modified junctions.