The significance of the retinoblastoma gene (RB) in the development of human breast cancer remains unclear. In the present study, loss of heterozygosity (LOH) in RB was found in 26% of 90 informative primary breast tumors and was correlated to DNA nondiploidy, a high S-phase fraction, and LOH at chromosome 17p13.3. However, allele loss was not associated with loss of RB protein (pRB) expression. Low to absent levels of pRB were found in 15% of 73 immunoblot analyzed tumors, most of which manifested retained heterozygosity in RB. Conversely, tumors exhibiting LOH showed often high pRB expression. Our data suggest that RB may be involved in the pathogenesis of some breast tumors, as evidenced by the absence of pRB, but that this alteration is acquired by mechanisms other than the unmasking of a recessive mutation by allele loss. LOH in RB may be merely a stochastic event in the unstable genome of aneuploid, rapidly proliferating cells or, alternatively, reflect the presence of an adjacent tumor suppressor gene.

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Supported in part by grants from the Swedish Cancer Society; the Mrs. Berta Kamprad Cancer Foundation; the John and Augusta Persson Foundation for Medical Scientific Research; the Hospital of Lund Foundation; the CTRF; the Thelma Zoega Foundation; the Gunnar, Arvid and Elisabeth Nilsson Cancer Foundation; University of Lund Medical Faculty Funds; the Social Society Funds; and the Swedish Council for Medical Research Funds.

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