STK11 and Gynecomastia: Early diagnosis of patients with pre-pubertal gynecomastia, including Peitz-Jaegers syndrome, defines the tactics of gynecomastia management and protocols for monitoring the development of other components of the disease in the future.This article describes two patients with pre-pubertal gynecomastia and Peitz-Jaegers syndrome with different molecular genetic defects: in one case associated with duplication of the STK11 gene site, in the other — with microdeletion of the short arm of chromosome 19 containing this gene.