Aphidicolin (APC)-sensitive fragile sites were identified in chromosome preparations from peripheral lymphocyte cultures of 12 four-way crossbred pigs. A χ2 analysis demonstrated that aphidicolin-induced breakage events and previously reported in vivochromosome rearrangement events were not independent. Comparison of expected Poisson and negative binomial distributions to observed breakage patterns indicated that the negative binomial distribution provided a better fit to experimental data. The negative binomial distribution is consistent with a distribution of breakage rates, i.e., non-constant rates of breakage at chromosomal loci across the genome