This track shows the gaps in the 26 Apr 2016 Mus musculus (house mouse)/GCA_001624745.1_NZO_HlLtJ_v1 genome assembly.
Genome assembly procedures are covered in the NCBI
NCBI also provides specific information about this assembly.
The definition of the gaps in this assembly is from the AGP file:
The NCBI document AGP Specification describes the format of the AGP file.
Gaps are represented as black boxes in this track. If the relative order and orientation of the contigs on either side of the gap is supported by read pair data, it is a bridged gap and a white line is drawn through the black box representing the gap.
This assembly has 49,204 gaps, with the following principal types of gaps: