What is Coffin-Siris Syndrome? Coffin-Siris Syndrome (CSS; fifth digit syndrome) was first reported in 1970 by Drs. Coffin and Siris to describe individuals with a shortened or underdeveloped fifth digit or fifth digit nail, along with developmental and learning differences, and a variety of organ-system issues. For many years, the diagnosis was made solely on clinical features; in 2013, the first genes associated with CSS were discovered. As more individuals with CSS are being diagnosed, the clinical picture has broadened; we are finding that not all individuals have the classic fifth digit finding, and that the extent of learning, developmental, and growth differences vary among children. Other terms that have been proposed for the syndrome include ‘BAFopathies’ and ‘SWI-SNF related disorders.’