I.by [美 baɪ, 英 bʌɪ]介词by often appears as the second element of certain verb structures in English (abide by, get by, stand by, etc). For translations, see the relevant verb entry (abide, get, stand, etc).
They started at the crime scene by genetically testing patients with pancreatitis and tracing the process, step by step, to the prime suspect -- that same CFTR gene.