The cyclin D1 gene CCND1 (a.k.a. BCL1 or PRAD1) is located in the chromosomal region 11q13.3 and encodes a regulatory subunit of cyclin-dependent kinases that promote progression through the cell cycle. The proto-oncogene CCND1 is amplified in a number of solid tumors including approx. 20% of all human breast cancer cases and about 30% of squamous cell carcinomas of the esophagus and the head and neck region. Amplification of chromosomal material from 11q13.3 harboring the CCND1 gene is discussed as a prognostic marker in terms of metastasis, tumor recurrence, and survival for several tumor entities. In gastrointestinal stromal tumors (GIST), CCND1 amplification was found in 16% of high-risk tumors and was absent in low- or intermediate-risk tumors indicating the prognostic relevance of this genetic alteration in GIST.