Fig. 1. ENU mutagenesis screen.

A behavioral assay was used to test for the presence of circadian rhythm mutations in firstgeneration offspring of ENU-treated males. (A) The mutagenesis procedure. Young male B6 mice were injected intraperitoneally with ENU at 150 mg/kg. On recovery of fertility, they were bred with B6 females to produce G1 offspring heterozygous for any induced mutations. Dominant or semidominant mutations could then be detected in G1 offspring. (B) Representative activity record of a wild-type G1 female from the screen. The record is double-plotted so that 48 hours are shown for each horizontal trace, and each day’s record is presented both to the right and beneath that of the preceding day. Wheel revolutions are recorded as pen deflections so that the dark regions represent times of activity. The animal was kept in LD14:10 for the first 20 days (illustrated by the bar above the record), then transferred to constant darkness (DD). Light pulses of 5 min were given on the days indicated by arrows. On transfer to DD, this animal exhibited a period of 23.7 hours. (C) Distribution of period among 304 G1 offspring tested for circadian phenotype. One animal, #25, exhibited a period that deviated from the mean by more than an hour. (D) Activity record of G1-25, the founder animal. This animal exhibited a period of 24.8 hours after 30 days in DD. (E) Distribution of period among 23 B6 N2 offspring from the founder animal. Thirteen offspring exhibited periods within the normal range, whereas 10 had periods longer than normal, comparable with that of the founder animal. (F) Representative activity record of a heterozygous male B6 N2 offspring. This animal had a period of 24.8 hours in DD.