Iojap (ij) is a recessive striped mutant of maize affecting the development of plastids in a local and position-dependent manner on the leaves. The ij-affected plastids are transmitted to some of the progeny even when the function of the nuclear gene is restored. Developmental defects during embryogenesis and leaf proliferation are other phenotypic characteristics of ij. The extent of striping and the degree of developmental arrest in ij depend upon genetic background. To understand the diverse and unique phenotypic expression of ij, a transposon tagging experiment has been conducted using Robertson's Mutator (Mu). A new ij mutant was obtained from crosses of the reference allele of (ij-ref) to Mu lines. Subsequent genetic and molecular studies showed that the mutant carried a new ij allele (ij-mum1) from the Mu lines and contained a Mu1 element that cosegregated with the iojap phenotype. A 6.0 kb EcoRI genomic DNA fragment containing the Mu1 element was cloned. ij-ref is unstable, and revertants (Ij-Rev) have been obtained. Using the flanking DNA from the genomic clone as a probe, DNA polymorphisms were detected between ij-ref and these revertants. Further, transcripts were restored to the normal level in Ij-Rev seedlings. Comparison of genomic DNA clones from ij-ref, ij-mum1 and Ij indicated that the ij-ref allele contained 1.5 kb of additional DNA related to a transposable element, Ds. Germinal and somatic revertant alleles were derived by excision of this 1.5 kb element from ij-ref. The structure of the Ij gene and the DNA sequence of its transcribed region were determined. The Ij gene encodes a 24.8 kDa protein that showed no significant sequence similarity with proteins listed in databases.