Nucleostemin is a newly identified protein that has been found within the nucleolus. Nucleostemin is a p53 binding protein that regulates the cell cycle and influences cell differentiation (page 88). As cellular differentiation progresses, the level of this protein decreases. The presence of nucleostemin in malignant cells suggests that it could play a role in their uncontrolled proliferation (Folder 3.2). In addition, DNA, RNA, and retroviruses and their viral proteins interact with the nucleolus and cause redistribution of fbrillar and granular materials during the course of viral infection. These viruses can use components of the nucleolus as part of their own replication process. Evidence suggests that viruses may target the nucleolus and its components to favor viral transcription and translation and perhaps alter the cell cycle to promote viral replication.
The nucleolus stains intensely with hematoxylin and basic dyes and metachromatically with thionine dyes.
That the basophilia and metachromasia of the nucleolus are related to the phosphate groups of the nucleolar RNA is confirmed by predigestion of specimens with ribonuclease (RNAse), which abolishes the staining. As mentioned above, DNA is present in the nucleolus; however, its concentration is below the detection capability of the Feulgen reaction. Thus, when examined in the light microscope, nucleoli
Cytogenetic testing is an important component in the di-agnosis and evaluation of genetic disorders and refers to the analysis of chromosomes. Chromosome abnormalities occur in approximately 0.5% of all live births and are de-tected in about 50% of the first trimester miscarriages (spontaneous abortions) and about 95% of various tumor cells. Chromosome analysis can be performed on periph-eral blood, bone marrow, tissues (such as skin or chorionic villi obtained from biopsies), and cells obtained from amni-otic fluid during amniocentesis. Studies of chromosomes begin with the extraction of whole chromosomes from the nuclei of dividing cells. These chromosomes are then placed on glass slides, hybridized with special fluorescence probes (FISH tech-nique), and examined under a microscope. A single fluo-rescent DNA probe produces a bright microscopic signal when the probe is hybridized to a specific part of a partic-ular chromosome. To obtain an image of all of the chromo-somes, a mixture of different probes is used to produce dif-ferent colors in each chromosome. Karyotypes labeled by this method allow cytogeneticists to perform a comprehen-sive analysis of changes in the number of chromosomes and chromosomal abnormalities such as additions or dele-tions. The paired chromosomes are numbered in the kary-otype, and the male sex is indicated by the presence of chromosomes X and Y (see Fig. F3.1.1a). The white box inset in Figure F3.1.1a shows the XX chromosome pair as it appears in the female. Sometimes, part of a chromosome will break off and at-tach to another chromosome. When this happens, it is re-ferred to as a translocation. Note that the red box inset in Figure F3.1.1a shows a translocation between chromo-some 8 and 14 (t8;14). It is clearly visible on this color image that a part of the original chromosome 8 (aqua blue region) is now attached to chromosome 14, and a small portion of chromosome 14 (red region) is now part of 
FOLDER 3.1 Clinical Correlation: Cytogenetic Testing