Multiple homologous sequences for the ferritin L subunit are present in mammalian genomes, but so far, only one expressed gene has been described. Here we report the isolation of a cDNA from a mouse bone marrow library, corresponding to an isoform of the mouse ferritin L subunit. This new subunit, that we named Lg, differs from the L subunit of ten amino acids. Specific amplification of mouse genomic DNA using the polymerase chain reaction (PCR) confirmed the presence of this Lg sequence in the mouse genome but also suggested that it must be encoded by an intronless gene. Using a series of different Lg-specific oligonucleotides as probes, we subsequently isolated a genomic clone containing an uninterrupted sequence, identical to the Lg cDNA. This Lg gene lacks introns and does not contain the 28 base pairs (bp) conserved motif usually present at the 5' end of most ferritin mRNAs, which confers translational regulation by iron. When transiently transfected into K562 cells, this Lg genomic clone is actively transcribed, suggesting that, although it possesses the characteristics of a processed pseudogene, it is likely to correspond to the gene encoding this new ferritin subunit.