In the naming scheme as originally developed, these substems mostly consist of a consonant, a vowel, then another consonant. The final letter may be dropped if the resulting name would be difficult to pronounce otherwise. Examples include "-ci(r)-" for the circulatory system, "-li(m)-" for the immune system ("lim" stands for lymphocyte) and "-ne(r)-" for the nervous system. The final letter is usually omitted if the following source substem begins with a consonant (such as "-zu-" or "-xi-"), but not all target substems are used in their shortened form. "-mul-", for example, is never reduced to "-mu-" because no chimeric or humanized antibodies targeting the musculoskeletal system ever received an INN. Combination of target and source substems results in endings like "-limumab" (immune system, human) or "-ciximab" (circulatory system, chimeric, consonant "r" dropped).