SFTPB and cystic fibrosis: SFTPD SNP rs721917 is associated with CF through 5 intergenic interactions with SFTPB (n = 3) and SFTPC (n = 2); rs2243639 is associated with CF also through 5 intergenic interactions but four of these are with SFTPB and only one with SFTPC (Figure 1B).