The Hus1 cell cycle checkpoint protein plays a central role in genome maintenance by mediating cellular responses to DNA damage and replication stress. Targeted deletion of mouse Hus1 results in spontaneous chromosomal abnormalities and embryonic lethality. To study the physiological impact of Hus1 deficiency in adult mice, we generated a conditional Hus1 allele, Hus1(flox), in which exons two and three are flanked by loxP sites. Cre-mediated excision of the loxP-flanked region produces Hus1(Delta2,3), which is capable of encoding only 19 of 281 Hus1 amino acids. Germline homozygosity for Hus1(Delta2,3) resulted in mid-gestational embryonic lethality that was indistinguishable from that caused by an established null allele, Hus1(Delta1n). Hus1 was inactivated in adult mice using a transgenic strain in which Cre is sporadically expressed in a variety of tissues from the Hsp70-1 promoter. Conditional Hus1 knockout mice were produced at unexpectedly low frequency and, unlike control animals, demonstrated limited inactivation of the conditional allele, suggesting that Hus1-deficient cells were at a strong selective disadvantage in adult animals. However, viable conditional Hus1 knockout mice consistently showed the greatest degree of Hus1 inactivation specifically in lung and mammary gland, highlighting varying requirements for Hus1 in different tissues. The novel tools described here hold promise for elucidating how the Hus1-dependent checkpoint mechanism contributes to chromosomal stability, DNA damage responses, and tumor suppression in adult mice.