|  |  | USP 15 |  |  |
 |  | wt | het | null |
Total
|
 | wt | 23 | 29 | 17 | 69 |
USP4 | het | 21 | 35 | 11 | 67 |
 | null | 8 | 22 | 0 | 30 |
 |
Total
| 52 | 86 | 28 | 166 |
- Mice heterozygous for inactivating mutations of both USP4 and USP15 were mated, and progeny genotyped by polymerase chain reaction analysis. The number of progeny of each genotype is indicated, where wt represents homozygous wild-type, het represents heterozygous, and null represents homozygous null mice. The absence of progeny null for both USP4 and USP15 is statistically significant (p = 0.000022)