Predicted spacing of the segments within chromosome 7. A CDF for the physical distances between the p-termini of the primitive segments of the microrearrangement model is presented here, on the basis of the assumption of an underlying exponential distribution, and the parameters derived from the best fit of the PG CDF to the chromosome 7 data. Granted these assumptions, about 50% of the primitive segments should be separated by distances of 100 kb size or less. If the amounts of intervening DNA between adjacent primitive segments could be considered negligible, then this plot would correspond to the size distribution of the primitive segments. Alternatively, under the gene cluster model, this CDF would correspond to the physical distances between gene cluster sites. Insert: Frequency Histogram for the Number of Genes per Primitive Segment. The parameters provided from the PG model were used to estimate the frequency distribution of genes within the primitive segments of the microrearrangement model. More than 40% of the primitive segments would be expected to contain no recognizable gene structure, and somewhat more than 20% of segments would contain only one gene. Under the alternative gene cluster model, this histogram would represent the frequencies of the number of genes per cluster.