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The kalilovariants of Neurospora contain a cytoplasmic genetic factor that causes senescence. This factor is a 9.0 kb transposable element (kaIDNA) that lacks nucleotide sequence homology with mtDNA and is inserted into the mitochondrial chromosome, often at sites located within the open reading frame in the intron-DNA of the mitochondrial 25S-rRNA gene. Genomes containing the “foreign” DNA insert accumulate during growth, and death occurs as the cells become deficient in functional large and small subunits of mitochondrial ribosomes. The kaIDNA transposon may be an “activator” element that causes breaks in mtDNA. Nonsenescing [+] strains of Neurospora do not contain kaIDNA.