In Zhuyin, not only is one of the tones unmarked; but there is a diacritic to indicate a lack of tone, like the virama of Indic.
ɪn ʒˈaɪɪn, nˌɑːt ˈoʊnli ɪz wˈʌn ʌvðə tˈoʊnz ʌnmˈɑːɹkt; bˌʌt ðɛɹ ɪz ɐ dˌaɪəkɹˈɪɾɪk tʊ ˈɪndᵻkˌeɪt ɐ lˈæk ʌv tˈoʊn, lˈaɪk ðə vɜːɹˈɑːmə ʌv ˈɪndɪk.