C2 cells depleted of their mitochondrial DNA were fused with synaptosomal fractions from brain tissue of inbred C57BL/6J mice. One clone possessed a 4696bp deletion that spanned from mt-Tk to mt-Nd5 and encompassed 6 tRNA genes and 7 structural genes. This deletion arose spontaneously and does not have direct repeats surrounding the breakpoint. This allele is maintained within cells that contain 30% mutant mitochondria. (J:65025)