The Cauchy problem for the Hamilton-Jacobi equation, which appears in molecular biology for the Crow-Kimura model of molecular evolution, is considered. The state characteristics of the equation that start in a given initial manifold bounded in the state space stay in a strip bounded in the state variable and fill a part of this strip. The values attained by the impulse characteristics on a finite time interval are arbitrarily large in magnitude. We propose a construction of a smooth extension for a continuous minimax/viscosity solution of the problem to the part of the strip that is not covered by the characteristics starting in the initial manifold.