Fig. 1.

Telomerase lengthens critically short telomeres in the absence of Atm. (A) Diagram of the Atm^+/− mTR^+/− × Atm^+/− mTR^−/− G5 intergenerational cross. Vertical bars represent relative telomere lengths. Progeny from this cross inherit half long and half short telomeres, giving littermates equivalent average telomere length. (B) Telomere length analysis of primary bone marrow from 8-wk-old animals. Cy-3-labeled PNA probes were hybridized to metaphase spreads, and telomere signal intensities were quantified as described (14). Each histogram contains data from three animals. Telomere lengths are shown as frequency distributions of telomere fluorescence intensities. Critically short telomeres are defined as those telomeres of the “zero” class of the distribution.