SMN1 and spinal muscular atrophy: This diverse distribution across the BIR domain family tree can be explained by genomic analysis of chromosome 5 which suggests that an inverted chromosome duplication is the reason for the genetic location of BIRC1 including the proximity to SMA. Alterations in this 1–2 Mb duplication are directly linked to spinal muscular atrophy (SMA) and BIRC1 has been suggested to have an SMA modulating role (Schmutz et al., 2004; Maier et al., 2007).