Ikaros, a transcriptional regulator of hematopoietic differentiation, is deleted in acute lymphoblastic leukemia (ALL) (Sun et al., 1999) and has previously been identified as a CIS in a screen conducted in transgenic mice expressing the Notch1 intracellular domain (NotchIC) (Beverly and Capobianco, 2003). Here, NOTCH1 is linked to in situ carcinoma.