Study of two families containing individuals with nephrogenic diabetes insipidus (NDI) indicated different types of 21.3 kb and 26.3 kb deletions involving the AVPR2 and ARHGAP4 (RhoGAP C1) genes. In the case of the 21.3 kb deletion, the deletion consensus motif (5Β’ -TGAAGG-3Β’) and polypurine runs, known as the arrest site of polymerase alpha, were detected in the vicinity of the deletion junction. Inverted repeats (7/8 matches), believed to potentiate DNA loop formation, flank the deletion breakpoint. We propose this deletion to be the result of slipped mispairing during DNA replication. In the case of the 26.3 kb deletion, the 12,945 bp inverted region with the 10,003 bp internal deletion was accompanied with the 2,509 bp deletion in the 5Β’ -side and the 13,785 bp deletion in the 3Β’ -side. We defined three deletion junctions in this rearrangement (DJ1, DJ2, and DJ3) from the 5Β’-side. The surrounding sequence of DJ1 (5Β’-CCC-3Β’) closely resembled that of DJ3 (5Β’-AGGG-3Β’) (DJ 1; 5Β’-cCCCgaggg-3Β’, DJ 3; 5Β’-ccccAGGG-3Β’), and DJ 1 was located in the 5Β’-side of DJ 3 without any overlapping in sequence. The immunoglobulin class switch (ICS) motif (5Β’-TGGGG-3Β’) was found around the complementary sequence of DJ 3. There was a 10-base palindrome (5Β’-aGACAtgtct-3Β’) in the alignment of the DJ2 (5Β’-GACA-3Β’) region. From these findings, we propose a novel mutation process with the rearrangement probably resulting from stem-loop induced non-homologous recombination in an ICS-like fashion. Both patients, despite lacking ARHGAP4, had no morphological, clinical, or laboratory abnormalities except for those usually found in patients with NDI.