TY  - RPRT
TI  - Letter: When is a null cell null?
AU  - P M Zeltzer
PY  - 1976
DO  - 10.1016/s0022-3476(76)80510-0
UR  - https://pubmed.ncbi.nlm.nih.gov/1084919/
ID  - 1084919
ER  -