**1. Introduction**

Chronic lymphocytic leukemia (CLL) is the most common malignancy in adults, and acute lymphocytic leukemia (ALL) is the most common pediatric cancer in western countries. These leukemic diseases affect the lymphoid line of blood cells. In most cases, the cause is unknown, hypothesizing that multiple genetic mutations and epigenetic changes are involved. Both diseases are vastly heterogeneous. While CLL is generally considered incurable and progresses slowly in most cases, ALL progresses rapidly and is typically fatal within weeks or months if left untreated. Historically, survival rates have been poor for patients with ALL. Since the introduction of chemotherapy, prognosis for childhood leukemia has improved greatly, and children with ALL are estimated to have a 95% probability of achieving successful remission. However, a total of 10–15% of patients still relapse despite undergoing intensive chemotherapy, and outcomes are far less encouraging in

adults. CLL treatment tends to focus mainly on controlling the state of the disease and its associated symptoms, rather than on its definitive eradication. The specifics of treatment will largely depend on the patient's prognosis and the specific CLL subtype. Therefore, lifelong observation and follow-up are strongly recommended and supported for all the patients. The combination of chemotherapy and nonchemotherapeutic drugs has improved survival of CLL patients overall, leading to long-lasting remissions. The pathology of CLL is complex in that it is influenced by a number of genetic and molecular changes, the CLL microenvironment, as well as various signaling pathways, of which the B-cell receptor (BCR) signaling pathway is central to CLL activation. Signaling pathways that are identified as being affected in CLL patients can provide opportunities for the development of diseasespecific drugs to the extent that they may be applicable in future clinical testing and molecular treatments. In any type of cancer, molecular therapy which targets specific regulatory proteins or their disease-associated posttranslational modifications can make way for novel applications which provide even higher specificity and efficiency with regard to treatment. This approach certainly applies to any type of leukemia.
