A patient with azoospermia and a Y isochromosome is described. The breakpoint producing this i(Y) was within the terminal short arm of the Y chromosome. Lymphocyte cultures from peripheral blood contained a high proportion of 45,X cells and cells with different Y-chromosome rearrangements. The i(Y) had either a monocentric or dicentric appearance. In dicentrics, anti-kinetochore immunofluorescence was present at both centromeres. However, this was also true for most of the functional monocentrics (pseudodicentrics). Kinetochore staining was generally positive at the site of the inactive centromeres; only a minority of the suppressed centromeres had lost their antigenic properties. Permanently growing lymphoblasts consistently showed a monocentric i(Y) with only one fluorescing kinetochore; the immunonegative Y centromere did not recover antigenicity.
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