Reference:
S. Grzesiek and A. Bax (1993) J. Biomol. NMR 3 185-204. (Link to Article)

Minimum labelling: ¹⁵N, ¹³C

Dimensions: 3

This experiment is similar to the CBCAC(CO)NH: magnetisation is transferred from ¹Hα and ¹Hβ to ¹³Cα and ¹³Cβ, respectively, and then from ¹³Cβ to ¹³Cα. From here it is transferred first to¹³CO, then to ¹⁵N^H and then to ¹H^N for detection. The chemical shift it not evolved on any of the carbon atoms. Instead, it is evolved on the ¹Hα and ¹Hβ, the ¹⁵N^H and ¹H^N. This results in a three-dimensional spectrum with one nitrogen and two hydrogen dimensions.

This is a useful spectrum for obtaining Hα and Hβ assignments, but isn’t necessarily a must.