In 2001, a second locus, SCKL2, was identified on chromosome 18 (18p11.31-q11.2), and, in 2023, Mudassil et al. identified the RTTN gene on the same chromosome 18 (18q22.2), whose mutations can also cause Seckel syndrome [15]. The gene discussed is RBBP8; the disease is microcephalic primordial dwarfism.