Scientists have characterized a new delayed-early response mRNA encoding a 21-kDa product (MRPL12) that accumulates during the G1 phase of growth-stimulated cells. MRPL12 is the mammalian homolog to chloroplastic and bacterial L12 ribosomal proteins. MRPL12 proteins are associated in vitro and cofractionate with ribosomal structures, as is the case for prokaryotic L12 proteins. Expression of a dominant inhibitory truncated protein leads to a severe reduction in cell growth by inhibiting mitochondrial ATP production. MRPL12 is the first mammalian mitochondrial ribosomal protein to be characterized. The human MRPL12 protein is encoded by a unique gene located on chromosome 17 (q25-qter). As no predisposition to colon cancer linked to this chromosomal region was hitherto reported, the MRPL12 gene might be involved in the process of differentiation of colonic epithelial cells.