Maf is a family of oncogenes which encodes a nuclear bZip transcription factor protein and has been originally identified from the avian oncogenic retrovirus, AS42. Maf genes have been reported to have critical roles in embryological development and cellular differentiation. In this study, in situ hybridization with S-35-labeled antisense riboprobes was used to investigate the distribution of c-maf mRNA in balb/c mouse kidneys from 12 (E12) through 17 days (E17) of gestation and then 1 and 4 weeks after birth. Immunocytochemistry of 4-week-old mouse kidney using anti-c-maf antisera was also performed. Kidney and liver sections from c-maf knockout mice at 4 weeks were stained with hematoxylin-eosin, and their histological features were examined. Expression of c-maf mRNA was first detected on E16 in the renal proximal tubules, and it was expressed through 4 weeks after birth. In the c-maf knockout mice at 4 weeks the cytoplasmic volume of the proximal tubule and liver cell was smaller. These findings suggest that expression of the c-maf gene may be involved in the embryological development and/or cell differentiation of kidney and liver cells. (C) 2004 Elsevier Inc. All rights reserved.