We report on two patients with mosaic tetrasomy of 8p[46,XY/47,XY,+i(8p)], a previously unreported cytogenetic anomaly. The first patient had a low percentage of tetrasomic (secondary trisomic) cells in lymphocytes and fibroblasts, an only mildly abnormal phenotype, and a rather benign clinical course. The second patient had a considerably larger percentage of tetrasomic cells in lymphocytes and fibroblasts, and had more severe congenital anomalies that led to his death at 8 months. A characteristic phenotype +i(8p) is suggested but not yet established. The manifestations of these two patients resemble those of mosaic trisomy 8 and mosaic trisomy 8p, with rib and vertebral abnormalities, absent corpus callosum, and enlarged cerebral ventricles.
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