women is higher than in men and that the number of micronuclei increase until around 70 years of age. Micronuclei levels ranged from 0.5 to 1.4% in men to 0.9 to 1.8% in women. Gender-related differences were mainly seen in younger age groups (<= 50 years) with an almost two-fold difference between men and women. The patterns in the number of micronuclei after 70 years of age is controversial. Some studies have shown that in individuals over 70 years of age, micronucleus frequency increases in both sexes. On the other hand, other studies have found that in the oldest age groups, micronuclei frequencies level off. The deficiency of micronuclei in some of the oldest age groups may be explained by the fact that micro nucleated cells are preferentially eliminated by apoptosis. However, higher micronuclei frequency corresponds to a decreased efficiency of DNA repair and increased genomic instability, which are typical in older subjects. Age-related increases in micronuclei frequency also correspond well with age-related increases in the hypoploidy and the age-related increase in sex chromosome loss. Alternatively, the leveling off of frequency of micronuclei in older subjects would suggest a threshold of genomic instability that cannot be crossed if the person is to survive. If this were the case, women appear to reach this threshold faster than men. Sex chromosomes contribute to the majority of chromosome loss events with increasing age. In females, the X chromosome can account for up to 72% of the observed micronuclei of which 37% appear to be lacking a functional kinetochore assembly possibly due to X chromosome inactivation. Multiple studies have shown that the frequencies of autosome-positive micronuclei in both genders and of sex chromosome-positive MN in men were similar and remained unchanged in older groups while the frequency of X-positive MN in women was higher than the average frequency of autosome-positive MN and continued to increase until the oldest age. The frequencies of chromosomal aberrations, damaged cells, and micronuclei are significantly higher in smokers than non-smokers. In normal people and many other mammals, which do not have nuclei in their red blood cells, the micronuclei are removed rapidly by the spleen. Hence high frequencies of micronuclei in human peripheral blood indicate a ruptured or absent spleen. In mice, these are not removed, which is the basis for the in vivo micronucleus test. See also Macronucleus Cancerous micronuclei Microchromosome References Organelles Ciliate biology