Next, we examined transgenic samples containing the insertion of elements foreign to the mouse genome. Since these sequences are not native to the mouse genome, the associated ChIP-seq reads were not expected to align to the mouse reference genome. These unmapped reads were thus realigned to a new reference sequence collection corresponding to the exogenous inserted sequence (Section 2). If these reads align to this custom reference sequence, then the presence of the sequence has been identified in the sample of interest.