In contrast, mammary carcinomas of F1 HER2/Delta16 mice exhibited three alternative patterns of expression: tumors expressing high levels of both full-length HER2 and Delta16, tumors expressing high levels of full-length HER2 and little, if any, of Delta16 and tumors with a low level of full-length HER2 and a high level of Delta16 (Figure 2A). This evidence concerns the gene ERBB2 and breast carcinoma.