Mosaic loss of chromosome Y (mLOY) also known as loss of chromosome Y (LOY) occurs to many men in some cells due to aging. Instead of the expected 46,XY karyotype, the affected cells have a 45,X karyotype due to the loss of the Y chromosome.^[1]

The occurrence of LOY was discovered in 1963.^[2]

"Age, genetic variants, ChrY structural aberrations and environmental stressors" such as smoking tobacco are all risk factors for developing LOY.^[1][3] The prevalence increases exponentially with age^[3] and more than 40 percent of men over 70 are affected.^[1] Unlike loss of autosomal chromosomes, loss of sex chromosomes except the one active X chromosome does not typically cause cell death. Elderly women also experience mosaic loss of chromsome X, but it is less common than LOY.^[1]

LOY in a small proportion of leukocytes (white blood cells) has been associated with a number of diseases such as cancer, Alzheimer's disease, and cardiovascular disease. Why this is the case is not understood.^[4] LOY can also occur in cells of the buccal mucosa and dorsolateral prefrontal cortex.^[3]