Cysteine catabolism is dependent upon cysteine dioxygenase (CDO), a thiol dioxygenase encoded by the gene CDO1. The clinical literature and, more recently, the study of CDO polymorphisms in disease and control populations have illustrated a strong association of impaired metabolism of cysteine to sulfate and taurine and/or CDO loss-of-function mutations with a variety of autoimmune and neurodegenerative diseases. The Stipanuk research group has recently generated a germ-line CDO knockout (CDO-/-) mouse that clearly has metabolic and phenotypic abnormalities. Phenotypically, CDO knockout (null) mice exhibit signs of connective tissue and skeletal abnormalities, especially joint hyperlaxity (hypermobility) and wry nose (contortions in the nasal bone). Our studies indicate that CDO null mice have clear connective tissue abnormalities. Key abnormal features include significant increases in lung air space, measured by mean linear intercept; lung elastic fiber disarray and entanglement; significant increase in matrix metalloproteinase-12 (MMP-12), an enzyme involved in degrading elastin, in the lung; and skeletal development retardation.