Gene expression profiling allows us to look into a tumor cell to see how genetic information is communicated and affects cell behavior. Data gathered during the development of the 70-gene assay (MammaPrint) set the stage for future assay development. Recently, a 21-gene assay (Oncotype DX) was recommended by ASCO and NCCN as a standard of care for newly diagnosed, stage 1 or 2, node-negative, estrogen-receptor-positive breast cancer patients. Treatment based on molecular profiling of tumor is advertised however there are very limited clinical data supporting this approach so far. Only one, relatively small, randomized clinical trial (SHIVA) have not met its primary endpoint - prolongation of PFS. Some other unpublished series were reported during ASCO 2017 and are discussed in this review. There are many issues to be resolved before the tumor profiling will enter the clinical practice with significant benefit for patients, eg. spatial and temporal heterogeneity of tumor cells in individual patient, wide access to targeted therapies, toxicity of combined targeted therapies. Implementation of imatinib, trastuzumab and vemurafenib to clinical practice has opened a new era of genomic-driven medicine in oncology [1–7]. Imatinib in chronic myeloid leukaemia (CML) prolongs overall survival (OS) of the treated patients to the level of OS reached by the general population. Trastuzumab in HER2-expressing breast cancer allows for a 35–50% reduction of mortality in the adjuvant setting and a 20% reduction of mortality of metastatic patients. Vemurafenib in BRAF-mutated melanoma patients extends survival but unfortunately most patients have progression after several months of response. These 3 drugs were taken as exemplary milestones in precision oncology because they have paved new roads but also because they have shown us some important limitations of targeted drugs. Imatinib is extremely effective in CML where the target – bcr-abl kinase – is a real driver oncogene and CML cells are oncogene-addicted; if bcr-abl kinase is switched off by imatinib, the CML cells are not able to survive. However, in contrast to CML, solid tumours with the same genetic aberration are much less sensitive to imatinib because other known and unknown genetic alterations are driving their growth