In addition to COX7B, MLS has also been associated with mutations in HCCS, the holocytochrome c-type synthase, and in NDUFB11, a subunit of Complex I. Somatic mosaicism and the degree of X chromosome inactivation in different tissues could explain the variability of additional clinical phenotypes that accompany MLS, such as developmental delay, abnormalities of the central nervous system, short stature, cardiac defects, and several ocular anomalies [137]. This evidence concerns the gene HCCS and Global developmental delay.