Abstract: The var1 gene specifies the only mitochondrial ribosomal protein known to be encoded by yeast mitochondrial DNA. The gene is unusual in that its base composition is nearly 90 percent adenine plus thymine. It and its expression product show a strain-dependent variation in size of up to 7 percent; this variation does not detectably interfere with function. Furthermore, var1 is an expandable gene that participates in a novel recombinational event resembling gene conversion whereby shorter alleles are preferentially converted to longer ones. The remarkable features of var1 indicate that it may have evolved by a mechanism analogous to exon shuffling, although no introns are actually present.