The number of chromosomes is based on the number of mitotic sites, so to count the number of chromosomes is to count the number of mitotic sites. There are 23 pairs of chromosomes in human cells, including 22 pairs of autosomes and 1 pair of sex chromosomes, of which the sex chromosomes are categorized into X chromosomes and Y chromosomes, with XX sex chromosomes for females and XY sex chromosomes for males. While looking at the number of chromosomes, it is also important to look at the structure of the chromosomes. Generally, if the number of autosomes is increased or missing, the patient may have low intelligence; while when the number of sex chromosomes is increased there may be decreased fertility, changes in temperament and disposition. Abnormalities in chromosome structure, such as misalignment and mutation, may lead to cancerous changes in the cells of the body. When abnormal chromosome number occurs after routine chromosome examination in the hospital, attention should be drawn to it and, if necessary, immunofluorescence and other tests should be carried out to pinpoint the location and carefully observe the chromosome structure.