Nucleolar disorganization in skin fibroblasts from DBA patients. (A) Galleries of nuclei after codetection by FISH of the U3 snoRNA and the precursors to the 18S rRNA. Hybridization was performed in parallel with probes complementary to U3 (conjugated to Cy5) and to the ITS1 (conjugated to Cy3). (B) Distribution of the shape factor calculated for the nucleoli. This factor (4π.S/P²; S, surface; P, perimeter) reflects the structure roundness: Its value is equal to 1 for a circle and decreases for flat or irregular objects. The nucleolar shape is much more irregular in NOTS-A cells when compared with control cells (Student t test, P < .001).