Motivation and Aim:We study one piece-wise linear dynamical system which describes functioning of a gene network regulated by negative feedbacks in order to find conditions of existence and uniqueness of periodic regimes of its functioning and show existence and uniqueness of solution of an inverse problem of identification of parameters of this system. Methods and Algorithms: The approaches to modelling of similar gene networks, description of phase portraits of corresponding dynamical systems and detection of their periodic trajectories (cycles) are presented in [1,2]. For some other non-linear dynamical systems, similar constructions were described in [3]. Results: For positive parameters A, m, α, where A > α, we consider symmetric piece-wise linear 3D dynamical system dx dt = L(z) -mx; dy dt = L(x) -my; dz dt = L(y) -mz.(1)