Confocal optical section of a rat mammary tumor with nuclei in red and corresponding centrosomes. Animals were treated with methylnitrosourea (MNU) to induce carcinogenesis. The extent of centrosome amplification in tumors correlated with the carcinogenic state of the mammary gland. The tissue shown here displays ductal carcinoma in situ (DCIS) with 45% abnormal centrosomes. For details, see the article by Goepfert et al. on page 4115 of this issue.