Figure 3. Clustering of nearest-neighbor center-to-center distances. (A) Distribution of nearest-neighbor center-to-center distances between all ribosome particles in the data set (n = 248) (For statistical relevance, the data from the two tomograms presented in Fig. 1 were merged). (B) Normal probability density functions corresponding to the data set in (A) (μ, average; σ, standard deviation). The analysis underlines the presence of two main populations (red and green lines) corresponding to ribosomes in close contact, as well as two minor ones (blue and dotted lines) corresponding to isolated particles.