Determination of the null distribution. Three different conditions are evaluated in order to consider a normal component as part of the null distribution. (I) The median of the values from each component must have a difference larger than md. If this first condition is fulfilled, the procedure tests the following conditions. The normal component will not be part of the null distribution if: (II) the normal component is small and well separated, that is, the minimum of the absolute log-likelihood ratio of the expression values under the two components is larger than mlk; or (III) the normal component is small and largely spread out, that is, the standard deviation ratio is smaller than rsd.