Abstract <p>Inactivation some regions of chromatin, heterochromatinization, can be caused by heavy metal lead. Normal blood levels of lead in children and adults are within the 0.1–0.2 μM range. Higher levels of 0.5–5 μM are known to have deleterious effects on digestive, respiratory, nervous, renal and reproductive tissues. The study results indicated that after induced with PbCl₂ at a concentration of – 0.5 × 10^–3 M and 0.5&#xa0;× 10^–4 M with 24– and 48 h of incubation in the human lymphocyte culture the number of chromosome aberration per cell, were significantly higher (results of heterochromatinization) than in the control group. Exposure to PbCl₂ at a concentration of – 0.5 × 10^–4 M during 24-h incubation indicated the activity of chromatids 15 chromosomes entering into associations (15 &gt; 21 &gt; 22 = 14 &gt; 13) and the activity of chromatid in association of 15:15 chromosomes (15:15 &gt; 13:13 = 14:14 = 21:21 = 22:22) increased significantly compared to the control (21 &gt; 14 &gt; 22 &gt; 13 &gt; 15) (<i>p</i> &lt; 0.01 and <i>p</i> &lt; 0.001, respectively). The results obtained indicate that low dose (0.5 × 10^–4 M) of PbCl₂ reveals the activity of 15 chromatids (characteristic for Breast and Lung cancer) than high mutagenic dose (0.5 × 10^–3 M) when the activity of 15 chromatids is not observed. We suggest that the epigenetic activity of ribosomal cistrons of the 15 chromatid is associated with malignant growth. Our study of the activity of ribosomal cistrons of acrocentric chromatids in pathologies (Breast and Lung cancers, Down syndrome) is a new direction in medicine, indicating the contribute to the development of preventive, the diagnosis of diseases and defining a new treatment strategy in the future.</p>