def _parse_rgn_segment(cls, fptr):
        """Parse the RGN segment.

        Parameters
        ----------
        fptr : file
            Open file object.

        Returns
        -------
        RGNSegment
            The current RGN segment.
        """
        offset = fptr.tell() - 2

        read_buffer = fptr.read(2)
        length, = struct.unpack('>H', read_buffer)

        nbytes = 3 if cls._csiz < 257 else 4
        fmt = '>BBB' if cls._csiz < 257 else '>HBB'
        read_buffer = fptr.read(nbytes)
        data = struct.unpack(fmt, read_buffer)

        length = length
        crgn = data[0]
        srgn = data[1]
        sprgn = data[2]

        return RGNsegment(crgn, srgn, sprgn, length, offset)